Change Image Format of a Screenshot
You can use the following Terminal command to change the default screenshot format (I'm reading that GIF and PDF and some other formats are also supported):

defaults write com.apple.screencapture type jpg


(You can also set the location key to a path to a directory in which you'd like the screenshots to be saved, as opposed to your Desktop.)

Also, the screencapture command can be used to save a screenshot to a specific filename, and configure more options (such as window shadows, timed capture, etc.).