Thanks for the post, but this does not seem to work for me.

What are your cache settings on Joomla Global Configuration and in the SIGpro plugin and the System Cache plugin?

It seems when I enable the system cache plugin a lot of my images (not in sigpro) don't load.

If you can include screenshots, it would be much appreciated.  :)

Global Configuration, Server tab Cache - Yes Cache Time 700 Minutes - Cache Handler File

SIG Pro plugin - Thumbnail cache expiration time 120

System Cache plugin enabled - Use browser caching Yes - Cache Lifetime 720

The order of the system plugins is important.

1. All other system plugins
2. SEF Plugins
3. System Cache Plugin

I have the System SEF & the System sh404SEF plugins enabled.

Thanks Yaani for the response, but with your suggested settings, my thumbnails don't show at all.  I'm trying to get assistance from joomlaworks support, to see if they can locate the problem.