GIFs used in TOC are not displaying

All,

We transfer the help files that RoboHelp generates to a Unix system.

For the last several releases of our product (RH6 and RH8), the GIFs that are used in the TOC (wht_toc1.gif, wht_toc2.gif, etc) are not displaying/rendered in Mozilla and FireFox.

We are having to delete all the newly generated GIFs and copying the GIFs from previous generated RoboHelp projects.   This is not a showstopper, but it is annoying.

Does anyone know what may be the problem?

Thanks,

Eric