Some possible problems:
The image servers are down or having other problems with their display caches.
You are blocking the images with your browser.
You have Remote Content turned off in your e-mail client.
Your proxy server is blocking the images.
The images aren't really images, but instead something like Adobe Flash items.
There are too many objects on the pages that you're trying to view, and so you're running out of memory for your browser.
You have changed the file association for JPEG or GIF images in your browser, so that they don't display inline.
The Webpages and e-mail messages in question are not compliant with HTML standards.
I have had all of these problems before, and any of them will probably cause the behavior that you are noticing.