Right click on the image (where it came from originally) and go to properties. Near the bottom it will give you the direct url for the photo, it has to end in .jpg to work.
The one I was trying to link ended in .jpg/image_medium which is, I think, why it didn't work. I think if you use a picture from google images and move it to one of your accounts like photobucket, it might then appear with just the .jpg url but doesn't mean it will work.