FooGallery Responsive Gallery – How to add different thumbnail images?

[ Moderator note: moved to How-to and Troubleshooting. ]

I am using the FooGallery responsive gallery and want to replace the automatically generated thumbnails with different images.

Is there a way that I can do this within the plugin, or is there an addon that will allow me to do this?

OR, where in the directory are the generated thumbnails located so that I may manually replace them with a like named file in the file manager?

I would really appreciate help for this. Thank you in advance.