Custom Character Texture, where do I start.

Hey I'm new here. I have this game in development and I want players to have the option to edit a texture map and upload it to their character in game. I've been looking around and I haven't been able to find a good tutorial on how to do. It also doesn't help that I don't know what to call this kind of thing. Can this be done in game by selecting a png or jpg, or does the player have to go into the game files and manually add it to a folder. Any help would be greatly appreciated. Thank you all for your time.