[theOZer]

6piston,
which vB version?
vB templates or other?
other issues, like, are you running just vB or do you run vBSEO, whatever?
If you have Manage Attachments turned on, and if in the first thread post and the user used vB Manage Attachments to load an image this mod will display that attachment's thumbnail, if coded to do so. The mod's trick is an outer join that relies on the fact that there's a thread first post id that can find/grab the associated attachment postid, enabling the outer join, piggybacking the attachment's row info onto the select.
Create a testvb. Do all your testing there.
After I set up a clean/virgin vB383 testvb, I put together a 'My Way to Install Thread Thumbnails' write up as I updated this testvb to use thread thumbnails in forums/new posts, searches, and tag searches. PM me if you'd like a pdf of this write up.
Ciao, Oz