There's no way to submit your own custom made image for an NPC that I know of. The only way to change an NPCs image is to go to the NPC inventory, find the NPC that you want to change the image of, and click Edit NPC to the right, and pick from the predetermined images that are there.
You can change the custom image normally the same way as for any other entity. The ID it asks for can be seen when you mouseover the checkbox to the left of the NPC in your inventory. It is not the name, but the ID number.
The only thing you can do is go to Edit NPC beside that NPC in your inventory and change its infofield. That will display on the NPC inventory, and you can enter anything you want there. That is the closest thing to a description. There is no place to enter a bio for an NPC, though.