As your image collection inevitably grows huge, you will need a good program to view, sort and manage your pictures.
Sadly, a lot of the image-viewers available for Linux today are crappy, buggy piles of dung. But a lot of projects evolve quickly.
Here are some brief descriptions of the alternatives so you can pick what sounds least crappy according to your needs and preferences.
 GUI image viewers
 Designed for KDE
KDE is a desktop environment. You do not need to use KDE to run KDE software, but you do need to have the required KDE libraries installed.
digiKam is a simple digital photo management application for KDE, which makes importing and organizing digital photos a "snap". The photos can be organized in albums which can be sorted chronologically, by directory layout or by custom collections. An easy to use interface is provided that enables you to connect to your camera and preview, download and/or delete your images.
Because it focuses on organizing that is where you find all the features. Editing is still premature, if not plain broken. But you can assign external editors, so the Gimp will save your day.
Konqueror, the KDE file-manager, has two folder viewing modes for images: One called "photobook" and one called "slideshow". Both of these allow you to browse through folders with images, but that is it. Both of these modes are totally buggy and Konqueror will crash if you delete images or move them by dragging them between two Konqueror windows. Both of these modes are basically garbage as of KDE 3.4.2.
KSquirrel is an image viewer for KDE with a disk navigator, file tree, thumbnails, extended thumbnails, dynamic format support, DCOP interface and KIPI plugins support. 44 image formats are supported. The current version is 0.7.0. KSquirrel understands many keyboard keys in image window, such as Page Down (next image), Page Up (previous image), +/- (zoom), * (fit image), and many many other. KSquirrel understands DCOP messages, see README for more. Since 0.6.0-pre9 such file formats as TTF, MNG, JBIG are supported. KSquirrel is the only image viewer with dynamic format support. It requires ksquirrel-libs package to decode/encode images. See screenshots for more.
Kuickshow is a KDE image viewer which allows you to browse images. It can also show you a slideshow, but the slideshow only supports one folder — Kuickshow lacks basic features like recursive slideshow viewing. Kuickshow is useless trash when it comes to sorting and organizing images. It gives you one window with files and folders listed, another where the images are viewed. The context menu is completely different for these two: You must use the file-manager window if you want to delete a picture, and you must use image viewer window to rotate or flip images.
Kuickshow is OK for browsing the images of one folder at a time, but that is also the scope of its usefulness.
SuSE 9.3 YaST story is that Kuickshow is somehow "obsoleted". Objective review of the evidence shows that this story does not hold water. It is part of KDE 3.5.x and YaST story regarding Kuickshow, if it is anywhere close to true, is limited to SuSE 9.x.
KView is a KDE image viewer which supports "image lists". It's the image viewer of Konqueror. It can delete files, supports slideshow viewing of the "image lists". When opened with one image you can browse through the directory with next/previous button. You can freely assign shortcuts for a lot of features. You can delete the image you are viewing. The full-screen option shows ugly borders which I could not remove.
It's not as fast as Kuickshow and the full-screen is ugly, but you can delete the files while viewing them and the "image lists" are a nice feature. But that is all.
Gwenview is another KDE image viewer and organizer. It allows you to browse through your images and view a single folder at a time as a slideshow. It also allows you to do simple things like rotate and resize images. And you can drag images between two Gwenview windows and organize your files without the program crashing (unlike all the other KDE programs and plugins). And the "del" key actually works (unlike Kuickshow) when viewing images (But it does pop-up a dialog asking for confirmation when you delete folders even if you have turned delete-confirmation off under Settings -> Configure -> File Operations). Gwenview is all in all a OK, but not good or even close to great, program for making simple changes like rotation to a huge set of files.
 Designed for Gnome
 Eye of Gnome
Eye of Gnome (eog) is a more or less featureless Gnome image viewer. You can browse a folder by going to File > Open Directory or doing "eog ./" from the command line while inside the directory you want to view. However it doesn't allow to "browse" multiple directories at a time (like in a tree form). It can display Exif information in a right "frame" when viewing a directory. This is useful for comparing shots taken with different speeds and or f-stops. For the most part this is a very simple application and good for a quick view of folders with pictures and displaying their Exif data.
GQView is a fair Gnome image viewer which has one useful feature: It is great for viewing all the images in a folder or folder tree. GQView allows you to flip, rotate and zoom images. The slide show feature, while being the only reason to try Gqview, is limited to viewing the images only: It lacks basic things like showing you the name and path of the file viewed, and if you use any of the very few features available on the right-click menu during a slide show it breaks and you'll have to start over.
2006-01-10: A very useful function in GQView is the "Find Duplicate Image" which provides a GUI for finding similar images.
Pornview is a image viewer very much like GQView and it allows you to view image slide shows of one folder, but does not support recursive slide shows. Pornview lacks all basic features you can expect from such a program except zoom (and auto-zoom). The keyboard shortcuts listed in Pornview, like with the majority of Gnome programs, does not work. Pornview also has a context menu options for removing folders when browsing through files, but this also does not work. Pornview is an OK alternative for viewing slide shows of one folder at a time, but that is it — it can not be used to manage or sort images.
It's cool. It does everything one would expect from an image viewer. It is able to rotate or zoom and manage the files. It lets you browse into folders, generate thumbnails and has working keyboard shortcuts. It also lets you specify the images to be able to choose from by command line, which can be used as a workaround (using a combination of "find" and "xargs") to the lack for a recursive file listing (beware there seems to be a limitation to the number of arguments one may pass to a command). The File List can be hidden and (if a starting image has been selected in it) the next file will be displayed by simple keyboard or mouse events (Slideshow). Might be what thou art looking for.
 GUI is over-rated?
The "mouse" is a very popular invention and many people, including my self, like to use this device to do things on a computer. However, some people prefer the $ command prompt and view this whole GUI thing highly over-rated.
A very special veteran: Special license. Special look and feel. Multiple window layout. Basic file managing, extended file format conversion and image manipulation capabilities. No slideshow function. Worth a try.
feh is an command line controlled image viewer using the imlib2 library to open and manipulate images. feh has a large number of operational modes. The default mode is an image slideshow, showing one image at a time from those specified on the command line. Keys and mouse control may be used to move through them. It is great for recursive slide shows: The command "feh -zZFr -D 2 pictures/" will display randomly (z), in fullscreen (F), recursive (r) and zoomed to screen size (Z) the pictures in the folder pictures/ with a delay of 2 sec. It does lack some basic features like sorting by full pathname in recursive mode. However, if you want to display an image without borders on your Gnome desktop, feh will do it easily, and the man page will even list the option (-x).
XV and XZGV are cool. If that's not what you're looking for then GQview is the least crappy alternative for viewing slide shows of images (and also browsing them with the mouse-wheel. This essential feature is stupidly disabled by default, Edit -> Preferences -> Advanced -> At the bottom of that tab, "Mouse wheel scrolls image" to enable).
Gwenview is the least shitty alternative for making small changes to a huge set of images (for the simple edits where you do not need powerful, feature-rich image manipulation programs like Krita and GIMP.
If you want to open some exotic image format like PSD, WMF, XCF or OpenEXR you use KSquirrel