You can change the default file type for screen captures by using a terminal command. Every installation of Mac OS X includes the Terminal application in Applications > Utilities. Most major image formats are supported including PNG, PDF, GIF, TIFF, and JPG, we’ll go with JPG since that is a common type of web graphic:
defaults write com.apple.screencapture type jpg
Then you must kill the SystemUIServer for changes to take effect:
killall SystemUIServer