You can also resize images (and change contrast color balance, etc.) in Preview on the Mac -- look in the Tools menu.
FYI, OSX Lion has a different save model than all the previous Mac OS versions... it auto-saves in most cases, and keeps versions to revert changes. This is great for most files, but bad for images you want to post as packing all those versions into the file makes it BIG. So if you are running Lion, use File > Export... to save a fresh, smaller file when you are done.
/pRC