I can briefly summarize how to do this. All the codes and steps are available in the recent forums for specific topics.
1. Add a Yes/No field in links.def, if only a few of them will submit graphics otherwise proceed to step 2.
2. Add another field of "link graphic" in links.def
3. If you want this graphic to be linked to the external site then add another field "link url"
4. Add corresponding fields for link graphic and link url in the input forms, to collect data
5. Assuming you are using templates (I do not know what to edit in site_html.pl) in your link.html template, next to the "link" add <A HREF="http://www.<%link_title%>"><%link_graphic%></A>
I think that's it.