Open a new Finder window and go to Applications : Utilities : Terminal
Now you have to decide what format you would like to use as the default file format.
BMP (Windows bitmap), GIF, JPEG 2000 (what you chose), JPEG, PDF, PICT, PNG, PSD, SGI, TGA, TIFF
Enter the following command into the Terminal window:
defaults write com.apple.screencapture type image_format
for example if you would like to change to png as default file format then enter this...
defaults write com.apple.screencapture type png
The next time you take a screenshot it will be in the format you just set!