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.).

You can use the following Terminal command to change the default location that they are stored in, and then use a folder action to perform conversion and other actions if required.

defaults write com.apple.screencapture location ~/Pictures/

You can also use the free application TinkerTool to do that as well. It's under the General pane: (Note that you'll have to log out and log back in for the changes to take effect.)

(as noted by this comment, you don't have to log out and back in again, just use the Terminal command killall SystemUIServer)