So I'm trying to figure out how to add, I think it's java script, to my board so when threads are started with pics in them, users can mouse over the thread title and the pics will pop-up giving a preview of what's been posted.

I have attached an example:

sounds like a simple show/hide div. Look into JavaScript tooltips, as that might be what you're looking for.

I think by default, vBulletin just adds the standard title attribute to the thread title so it can't support images in that manner.

I've looked into using Overlib, but I'm not sure how to integrate it into Vbulletin. I've also seen a CSS version without javascript.