Like this, you mean? You need to insert it as an image. When you go to edit your signature, click on the button for "Insert Image" (it looks like a little picture of a mountain and a sun), then paste the URL for your picture into the box that pops up. Hopefully this will work.
EDIT: Ahhh, nevermind. It appears there is no Insert Image button in the Edit Signature text box. Not sure if it's possible to do it with HTML-type code... Ahhh, yes, it does appear to be possible. Try putting img tags around the URL. Like this <IMG>http://images.meez.com/user/4/7/6/0/8/8/4/4760884_bodyshot_300x400.gif</IMG>, but use square brackets instead of angle brackets ([ and ] instead of < and >).