Usually it's a matter of looking at the source code and figuring out what directory they are using for the image. Then simple append that directory onto the main site link and voila!
Here is an example (I put spaces after the < designator to prevent activating the link):
This is the URL for Urth Fist Santa Cruz:
http://members.cruzio.com/~cruzef This is the URL for the image I found there:
< img src="/pictures/efsymbolanimation.gif" width="277" height="364" border="0" align="bottom"> Here is what I post here:
< img src="http://members.cruzio.com/~cruzef/pictures/efsymbolanimation.gif" width="277" height="364" border="0" align="bottom"> Thusly:

They're fisting the Treeeee!!!