There is nothing wrong with using a Mac!!!!
In Photobucket you can get it to automatically resize your images to the 640 x 480px forum limit, by choosing the size in the dropdown box here.
Are your pics from a camera or phone? If they are too small to start with, it may be causing the problem, but I see Dixon seemed to be able to get ahold of a higher resolution picture...
Oh, and welcome to the forums!