I think you want to set up a Signature Alan; you do that in your Control Panel.
Copy the URL into your clip board. Type whatever it is you want to say like "My Photostream" and highlight it; then click on the globe thingy above the message pane, paste the url into the box that opens. You can then format the text as required, for instance colour, bold, etc.
Hope this helps, but come back if you get stuck.