As long as they're under 20MBs, you can upload GIFs, JPGs and PNGs of any width or height to Mocksup.
In addition to website mockups, Mocksup is the perfect place to share mobile and tablet mockups, because its admin supports iPhones and iPads, as well as Android, Blackberry and webOS devices.
So, let's say you are using Mocksup to share iPhone mockups with a client. If you uploaded a 320px wide by 480px high image via your desktop and then logged into Mocksup on your iPhone, your uploaded mockup would perfectly fill the screen. Link a few of those mockups together and you've got an instant mobile prototype.