Have a Skins folder where you could put your own custom skin in and than be able to use it, by Changing the Skin in your Save Profile Folder, and have a Downloadable Blank skin ( A blank canvas if you will ) where you can just edit in a simple Program most Computers have, Kinda like how Minecraft use there skins, everyone knows how to use and add them.
Just a suggestion
we have no way to serve the skin to other clients, you can swap your skin client side with content packs, compile an XNB of the sprite sheet, then you need a new folder in content packs, you make a folder called Textures and one called NPC inside there. this only affects how you see that skin on your end, other will see you as whatever your skin was
the game already does 'custom skins' but you and everyone you play with need a copy of it or they're going to crash when they try to draw you lol. you can drop a completely new sprite sheet in the /Content/Textures/NPC/ folder and change your skin in your /Save/charactername/Player.dat (its just a text file) so the skin in your save matches the file name for the skin you chose.
if you want your freinds to see the skin hand out the content pack, maybe do one that swaps a few of our skins to custom ones so you and your freind(s) all have custom skins in the same game.
Or like how Minecrafters apply thier skins. That would be a massive help to making each skin usable.
thats far easier said than done. MC has a master server with user accounts you can easily tie a skin to.
Yeah, but it's a program that does that for you. Instead of having to download the XNB Exporter, you could edit a template and put the finished product into the program, which does the rest.
we literally can't load things without the content pipeline, and you can't compile XNB files without XNA game studio, terraria modders have to deal with the same thing. thats just how it is.
edit: and some people will compile your sheets for you, it only takes a sec lol