When you upload to imageshack.us, it also makes a thumbnail for you to use of sites or forums.
If you don't want that, you can just use a resize feature that most scripts these days have. Just from top of my head, Adobe - Flash, Photoshop, Fireworks, Microsoft Office apps (specially Microsoft Office Picture Manager).
But as Josh here mentioned, using a gallery script or some sort of image manipulation script would be better, since thats what they're designed for and pretty much would automate the process for you.