The image thumbnails in my admin control panel are 600x600 px. They were smaller before, I believe 100x100 but somehow they changed and I cannot figure out why or how to change them back.

Please provide minimum info like what Zenphoto version.

It is the latest version:
Zenphoto version 1.4.0.4 [7122] (Official Build)

I've changed the theme quite a bit but have not done anything to the admin files. When the files were large 2mb or 3mb and bigger then the thumbnails in the admin panel were normal. Since then I decided to only use small versions of my files 900px max so I deleted all the old large image files and the cache then uploaded the new smaller ones. Since then the thumbnails in the admin panel have been rendering to 600px square. I did have the image display size set to 600px Height..

At least in the dev stream the back-end image thumbs are all custom 85x85 sized. I did not look at 1.4.0, so that might be difernt. But I know of no way that they could be 600x.

Here is a link to a screen shot:
http://www.warrendiggles.com/Picture-1.png

If I recall right this was a bug in 1.4.0.x releases that has indeed been fixed in 1.4.1 beta.