The default format for screenshots in OS X is png, which is not a particularly universal format. I prefer jpg because everything accepts it. To change the default to jpg, type the following code:
defaults write com.apple.screencapture type jpg
To change to any other format (i.e. gif, tiff, pdf, et cetera), replace “jpg” with the desired extension. To activate the change, log out, then log back in.
Edit: Sorry if there was any confusion, this is all done in Terminal

Mike
Omer Zach
Charlie