Hi simmo,
I've run into this several times when the pic has not completely downloaded but IE-6 thinks it has. If you use IE-6 and turn on the auto-resize option, you'll see that the resize-to-normal button, normally in the bottom-right corner of the pic, is in this case below what seems to the the bottom of the pic.
And IE will offer to save the part-picture, but can only do so as a BMP.
If you want to keep it anyway, even though it's not complete, just about any halfway-decent graphics program (I like Graphics Workshop) can convert it to JPG or any of many other formats -- you'll probably want to crop it afterwards to eliminate the "invisible" part of the pic and trim the file-size a little.
Best,
Mike
As a P.S., if the site's handy (or you're still there, better yet), try clearing your browser cache and then refreshing, sometimes that will succeed in getting the whole pic (and file) so you can save-as JPG to start with.