I'm having a problem getting Simple Image Gallery and SIGE to work on my slave sites. The plugin is unable to create/display the thumbnails for the images, although the full-size images display fine in the lightbox popup. I've seen several places on this forum where people say both of these plugins work fine, but it's not happening for me.

I'm thinking it's a problem with the path of the images directory relative to the plugins directory. I can get around it by copying the plugins directory to my slave sites instead of using a symbolic link, but this defeats the purpose of being able to maintain and upgrade the sites from one installation.

My directory structure is:
Master: /public_html
Slave: /public_html/slave1, slave1.com

All slave sites are defined as addon domains in cPanel and map to the correct directory.

Any ideas?

Perhaps you could use a browser debugger to check the path present in the images that are not displayed.
This could help you identifying where the images directory is read and why it is not displayed.