Why won't my cover show up in my signature?

Hi! I'm so excited. Because of the help of lovely AWers on this magnificent forum, I have now epubbed one of my novels. And I've already sold one copy! WOOOOOO!

Anyway, I want to be like all the other cool AWers who have their cover in their signatures, but so far I haven't been able to do it. I get a little red X where the picture should be.

I click on the insert picture button.
I copy and paste an url to my cover pic uploaded onto mediafire.
And no dice.
What am I doing wrong?

Any and all advice appreciated!

I figured it out! At least, I figured out one way. Kateness, your way probably works too. But there's a special spot below the text box that I was failing to read that allows for the upload of a signature picture. Yay!