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This is common and has happened to most of us I think - it certainly has to me. For some reason, NewBB2 takes 'a while' to detect the image processing applications that are installed on your web host. It took about 2 or 3 weeks for me, then, all of a sudden, they showed up. It shouldn't cause you any problmes in the meantime though.
GD is an image processing application that ships with PHP. It's the most basic of the three and was originally designed for GIF files. ImageMagick and NetBPM are more hardcore and better. I set my modules to use ImageMagick for no particular reason.