The X indicates that the picture cannot be found. If the pictures are stored locally on your PC, it means the src attribute (the path to the picture) is bad. If the pictures are stored remotely on a website, it could mean that the src attribute is bad, or that you don't have access to the picture (could be any number of reasons for this). Take the src attribute and paste it into the location bar on your browser. If you can see the picture, you should be able to see it in the TD.
When I say src attribute, I mean:
<img src="
http://www.google.com/intl/en/images/logo.gif">
If you post one of the <img> tags you've included in your layout it would be easier to help.