How caching works
There are several different caches:
Zenphoto does not touch the original full images you uploaded to the albums directory. But it does cache the resized images like thumbs, sized large images or any other sized images so they don't have to be generated again.
Zenphoto also caches sized images if they have the same size of the original full image. It does this to prevent overhead checking this everytime if it needs to be cached. If available on your server Zenphoto will use symlinks to those already sized instead of copying the image again (unless a watermark is set).
Zenphoto generally creates these cached files on the fly. This means they are not preliminary created but on request, e.g. if someone visits a page of your site. Depending on the dimensions of your images and your server's power this might result in an initial delay once. Other (...)