If you have a "?" mark in the link, as in Rosie Nell's link for example, the chances are the link is to a program, and the stuff to the right are parameters being passed to the program. To see if your link is good, you should be able to paste it into the address of your browser and it should display with no page, no descriptions, nothing other than the image itself... i.e., http://www.oldmerlin.com/img/Avatar_08.jpg
and you should be good. The link HAS to be directly to an image.
Before uploading, if you don't want to be squeezed or stretched, use an image that is cropped to a square aspect ratio, like 300x300 pixels. There may be an upper limit on the size for this forum to handle, but I've put up some that are 300 - 400 pixels square without issue.