I doubt you will be able to do anything with the photobucket site.
Some options:
1/ Make your users stick to a file size on photobucket.
2/ Resize your images using the width option in the html image tag - leave the height blank or if using php do a calculation with getimagesize.
3/ Resize the images dynamicaly as you say check out this thread: http://redux.imagemagick.org/discourse- ... lit=resize
there is a dynamic way using imagemagick although I am not quite sure how to use it in your case and a link to a .htaccess example that uses GD. You may get some ideas from that and search google for more .htaccess methods as there are quite a few methods.