[Erwin]

Do this:

1) In Admin CP, vBulletin Options, make sure no-cache headers is OFF.

2) Use RELATIVE PATHS for images, NOT absolute paths.

For image paths, do NOT use http://www.youdomain.com/forums/images

Use this instead:
images

To do this, in your Admin CP, under Styles, click on Modify, then in EACH of your styles (Default and customs),

Find this section and use the variables given:

Image Paths
Image folder path - images

Title image path - images/logo.gif

Reply image path - images/reply.gif

New thread image path - images/newthread.gif

Closed thread image path - images/threadclosed.gif

By using RELATIVE paths, your images will be cached. By using ABSOLUTE paths, your images will NOT be cached.