Over here I wrote about how to change the path where screenshots are stored. Another potentially handy option is to specify what file format you prefer for your screenshots. There is a surprisingly large number of choices: png, pdf, tiff, pict, jpg, bmp, gif, psd (PhotoShop), sgi (Silicon Graphics) and tga (Truevision targa).
You can change the file type by running the following defaults command from a terminal, where ‘format’ is one of the options above:
defaults write com.apple.screencapture type format
Logout/login for the new file type to take effect. As an alternative, you can run the following command from a terminal, however, I had problems getting this to work on a consistent basis:
You can also use the Property List Editor to set the your screencapture preferences. The file you need to open is located here: ~/Library/Preferences/com.apple.screencapture.plist (use Finder to locate the file and double click to open).