It's nice to see you again Hyzenthlay

Below each picture, there are a few icons. Select the one with the arrow pointing right "Share Photo". Select the BBCode tab, and you should get something like this:
- Code: on
1223171631c by Michelle Lloyd, on Flickr
(example taken from the “Your desktop” thread)
The part between the img tags is a direct link to the image; paste this link in the "URL to avatar" box in the "change avatar" page.
For a signature, copy the entire code (minus the part “by [name], on Flickr”). However, you will have to choose a smaller size for the image due to the signature policy.