I thought about making a system like this, and had a working script upload an image from the scriptfiles folder. But instead I took a different route:
Zodiac's system for uploading lets the player upload it to my webserver:
http://uploads.zodiacdev.com
I then have an DB-NPC check every 5 minutes for new graphics, or an Uploader can force a check. You can then use requesturl to get each file, and put it in the proper upload folder. After confirming it's been successfully uploaded on the server, it then deletes it off the webserver.
The benefits of using a webserver allows you to offload some work on the npcserver, provide a neat webform, as well as the ability to have the webserver set transparencies for you.