Ahh, okay, sorry

I get the question a lot, so I just went away at answering it like I usually do.
It'll take some thinking through... and since every dual-server configuration would be different, it would be very difficult. That's why I don't think it's worth spending much time on... I want to keep ZP simple and all that, you know.
It's not as simple as moving the directories and updating the database -- the processed images aren't even stored in the database. The image processor goes and looks for their existence on the filesystem first, and if they don't exist then it reprocesses them.
There would be many advantages to storing that value in the database once it's cached, but it would still require a file check just to be sure, in case the image has been deleted or something. I'd like to keep the dynamic nature of the image processing, and never succumb to the "do it all when uploading" way. I think the image processor is one of zenphoto's biggest strengths.
Anyway, to use files from a different server, they would have to be downloaded to the processing server, resized, stored somewhere (cached either remotely or locally) and the URL returned to the user wherever that final location may be. It would be useful (if we knew the processed images were never deleted) to store the cached URLs in the database somehow, so that there were no remote file operations for each image (which are orders of magnitude slower than local file ops).
It would be fun to find ways to optimize the whole process, but still, I don't think it's worth my time. If you want to try, go right ahead and keep us updated with your progress