Thank god, this has been annoying me for awhile. I've been using the thumbnails but people STILL insist on having their huge 1920x1200 picture straight on the forum.
Another advantage of the thumbnails is that less bandwidth to download the thumbnail, which means your picture will stay on Imageshack for longer, as every time someone just opens the thread, it doesn't download the whole image (just a tiny 3kb one). The pages will also load faster too.