Mike in Michigan wrote in complaining that while he loves the easy way to take screenshots in OS X, he didn't like that they saved as PNGs. Not that he has anything against PNGs, mind you, but most of the sites he visits let him embed JPEGs only. Well Mike, we can help.
Change default screenshot format
Open Terminal and type in:
defaults write com.apple.screencapture type IMAGE FORMAT.
The image format can be JPG, TIFF, PDF, BMP, PICT, or others. You can even change it back to PNG with this command, if you change your mind later.
Then log out and log back in again.
After that, any screenshots will be saved as the format you specified.