Thanks for your input Hitech.
Okay, I see what you want and why.
BUT, just to make it clear. I don't make the ZIPs. A skin is uploaded as ZIP by the skinner. I just control, that only ZIPs can be uploaded to limit bandwith.
It doesn't "just depend on how you make the zip."
PHP is very powerful. I found a tutorial how to upload files and archive them into a directory in a ZIP archive.
BUT, this hurts my brain. It's not meant for beginners like me.

I just took a small look at the script tutorial and it sounds pretty cool and useful, so I will try it, but I can't promise when and if I will understand it and be able to create the ZIPs on the server on the fly.
