I don't know the site offhand, but if you can find a post where someone has used the photo you want, just right click on the photo, click properties, and you'll see the URL address of where the photo is located.
If it's on a public site, you can just use the same URL.
If it's in someone's personal album or site, you shouldn't direct link to it. If the photo is not trademarked, you can right click on it, select save picture as, and put it on your own computer. Then upload it to your own internet server and link to it in your signature.