I think I wrote an Automator script that captured screens in another format, or perhaps it converted the one you just captured to some other format. It was something my wife needed to use with Powerpoint, if I remember correctly, and it worked for what she needed.
If you don't have a better solution, I could track down that script for you. But, knowing you can change image file formats in Automator might be enough of a clue if you want to do it yourself.