Forum Moderators: not2easy
Could be for a number of different reasons. Here are a couple of ideas ...
This is not the normal problem problem where I an viewing an old cached copy of my page because the text is being updated
This doesn't necessarily mean that the picture is not cached. Hold the shift key down when refreshing the page, or clear out your browser cache and try this again.
The site contains banners pointing to other sites
First thing I'd probably try is to save copies of the images to your own site, and link to them there. Maybe the sites serving the images have bandwidth issues ... this would prove/disprove this.