If there are new posts, to load an image to let users know that.
With phpbb, mybb, xenforo I can easily do that.
Why it can not be posssible with vbulletin?

Why wouldn't your images already be part of your global sprite sheet?

Thank you for trying to help me but I need solutions not questions.

That is a solution to your problem.

It is not.
Let me share you my way of optimising vbulletin:
> replace most images with css buttons
> combine a few of them to preserve the style

That is why it is imperious that I check if there are new posts or not.

You should serve 1 sprite, with all of your images, not multiple.

Yeah, well, that information is just not available out of the box. You can do it on forumhome or on forumdisplay for subforums, but Lynne has explained to you how to do that. As far as I know, inside the forum (i.e. forumdisplay for threads and showthread) that information is just not readily available. You'd probably have to do a plugin checking the lastpost date of the forum against the last activity date of the user. Maybe that information can be pulled out of the datastore.

Well, that is just great...

Lynnes suggestion works if you need to check if subforums have new posts.
But you can't check if a category forum has new posts using her solution.

Are you aware of a plugin that can solve my problem or some piece of code that I might insert in a template?

So here is what I don't get. you want to reduce the overall http request, why aren't you just serving one big sprite image, with all the graphics your forum uses?

Why even bother using graphics when you can use something like FontAwesome?