i don't know which desktop environment you use, but in kde you can actually use alt+printscreen and printscreen to take screenshots. after doing alt+printscreen or a printscreen, for example, open up kpaint and in the file menubar choose Edit > Paste Image.
a word of note though, for me printscreen actually sucks. using the above method works, but for some reason on printscreen the windows decorations will disappear until you relogin. alt+printscreen works fine though. but for full desktop screenshots, it's better to use either the gkrellm shoot plugin or some other screenshot app like ksnapshot. but alt+printscreen and printscreen works in kde. not sure about gnome.
here's a screenshot of kpaint with a screenshot of konqueror using alt+printscreen. it's big and uncompressed i think (140 KB).
Clicky clicky
edit: forgot to mention, you may have to configure your keyboard shortcuts in kde to use the printscreen key for desktop screenshot. i think alt+printscreen is a default key combo for a windows screenshot.