In OS 10.5 the screen captures are saved as PNG files by default. However, it’s possible to change them in a variety of formats including JPG and GIF. Here’s what David Passell recommends:
http://www.maclife.com/article/feature/terminal_tips_every_mac_usershould_know The article is "25 Terminal Tips Every Mac User Should Know."

Scroll down to tip #8 which gives directions to change file format for screenshots. (Note: this article continues for 5 more pages.)

After following those directions, David tells us to Quit the Terminal and restart your computer. Then try it out, using CMD-SHIFT-3 for the whole screen or CMD-SHIFT-4 for only the part that you scroll over. You should be able to find Picture n.jpg which launches with Preview. “Worked fine with me,” are his closing remarks. Thanks for the info, David, you’ve been one busy guy today!