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,.
Command-Shift-3 is the command that takes a screenshot in OS X (or Command-Shift-4, if you just want part of the screen). It saves the image as a PNG. In Mac OS X 10.4 and later, you can change the default screenshot format at the command line.
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.