if you're signed in.. you should just have to roll your mouse over the pic and the 4 links will show under the pic... choose the one on the bottom and copy it...
if you're NOT signed in... then i think you have to click on the pic, then when i comes up the links are on the right hand side, then you can choose the one on the bottom....
just cut, copy, and paste that link into your sig box
remember to check the size though