[Logician]

Not an easy hack because:

1- In simple way, to "know" whether a thread has images in it, vbulletin would need to load the entire thread into the memory to make a check in the thread listing, search page etc. and considering the fact that you have 40 threads listed in that page which might have say more than 400 posts in it, it would be a real performance killer.

2- In the harder (but wiser) way, to avoid such a check, you would need to "mark" the thread whenever it has a IMG tag in it. However in this case, you would need to hack a lot of sections in vbulletin to put this "mark" into the database. For example, new thread, new reply, edit post, admin thread moderation, moderators post editing etc.

So possible but not smooth or easy hack.