With the Zenpage theme, I cannot control the image size on the image page through admin/options/theme/image_size. No matter what I input under 'image size', it stays the same after saving. If I switch to other themes, the change 'image size' works as anticipated.

Also with the Zenpage theme, when I click on the image on the image page, instead of getting a full sized image, I get a rather scaled down image of the original. Again, this does not happen with the other themes.

Are these occurances inherent in the Zenpage theme or is something else going on that I am missing?

First, let me mention that different themes may be different setup. Some like the stopdesign theme are much more complicated and need more fixeds sizes for layout reasons than the default theme for example. Also the zenpage-default theme does a few things differently.

1. The zenpage theme actually uses hardcoded image sizes for layout reasons (580 x 580 for the sized image, as I felt the standard 595px too look to wide). As everything you can change the sizes on the image.php of the theme itself.

2. That is indeed the full size image . The thickbox script that is used for that is quite clever and scales the image to fit within the current viewport (browser window size). Again there is a theme option to disable the thickbox, then you will get the normal full image.