As a member you can also upload them directly.
Aidy has kindly produced a guide to doing so
here and while he mainly focuses on Avatars this method also works with attaching photos to threads. (when replying or making a post, in the box under the one you are typing in, is one that says 'Attach file' just browse to the filename) A good size is 640 x 480 pixels. You can also of course use any graphics program you are familiar with to acheive the resize or if using XP then you can use the resize
powertoy.