http://i.imgur.com/6QPlVjl.png
Just put that link into the button that it points an arrow towards for a test, if it works, do the same thing with any picture (gif, jpg, bmp, whatever) and it should work.
Deleted other post because yolo.
Also, i didn't say just put the link of the picture into your signature.. Press the damn "image" button then put it in there. :p
it should look something like [IMG]http://sdfdsgdsgdfgdf[./IMG] without the . or something.