Since you're a Mac person, "alt-print screen" = "command-shift-3". It'll make a PDF (!) of the screen and stick it on your desktop. If you just want to make a copy of a section of the screen, press command-shift-4 and drag the cursor like every other selection tool in every other paint program. Alternately, there's /Applications/Utilities/Grab, which will do similar things and save to TIFF files.
