OK,,, I've done some testing.
I am testing with a stock installation of ZP photo and a stock version of zpArdoise, no modifications. I have cleared the cache for testing to witness exactly what is generated when a page is loaded vs a pre-caching request.
Pre-caching with a ZP official theme works fine. Pre-caching with zpArdoise has many issues. zpArdoise had a new release on January 5, 2013. I'm wondering if I had a previous installation all images would have already been cached, and I would never experience these caching issues. Since I moved severs, I witness what happens when pre-caching entire gallery.
First off, if you select any zpArdoise size for pre-cacheing available in the selections, the only size that will be generated is the 40 x 40 pixel size, for admin. I have witnessed this by watching my FTP client.
Secondly, if you select an individual zpArdoise image size, but do not select the admin size box, and then click the "cache the images", the zpArdoise selection will be deselected, and "all admin sizes" will be selected and then nothing will happen.
Also, zpArdoise is registering a slideshow size of 450 but the actual cache size generated when used by viewer is 525. I am using Galleriffic option, so image caching needs to be aligned with gallerific sizes.
I am not positive this is distinctly zpArdoise problem, but I can't duplicate it on a ZP approved theme.
All testing on FF W7, no time to test other browser yet.