Ok, I have answered my own question:

This works for me in both detail.html and link.html

First I added a field to the links table called "thumb_image". Then this code added to the page:

<%if thumb_image%>
<img src="<%build_root_url%>/thumbs/<%thumb_image%>">
<%else%>
<img src="<%build_root_url%>/thumbs/default_image.jpeg">
<%endif%>


Then modify any link to include the image name in the field.

Good luck,

Rich