[Andreas]

Originally Posted by Gearloose

Do you mean, it is directory specific?
If so, should I put this in the directory of the images that I want to be cached or in the directory of the images that I want to not be cached?

Yes. You have to place this in the directory where you don't want images to be cached.

But as I already said, the images "never" change.
What actually does change are the image URLs within the HTML that is being generated by vBulletin (eg. <img src="...statusicon/post_new.gif" ...> and <img src="...statusicon/post_old.gif" ...> for example)
If you see wrong images, you most likely have problems with a (faulty) proxy.
To get around this, you would have to disable caching:

ACP / vBulletin Options / Cookies and HTTP Header Options / [X] User No-Cache Headers

However, this might/will increase bandwidth usage and server load.