What is the easiest way to convert a Jp2 image file (like photobooth takes) into a JPEG file? Something that actually changes the file, not just the extension I mean.
Photo Booth (if you are referring to the Apple program that works with iSight cameras) takes pictures in JPEG format, and - at least - on my Mac the images gets ordinary .jpg suffixes...
.jp2 files are usually JPEG 2000 files, and you should be able to open them in e.g. Preview and save them as ordinary JPEG files...