I looked into how difficult it'd be to make a Spelunky skin and turns out, not too hard.
Here's a link to a forum that seems to have all the skins/mods available collected into one location. There should also be a tool there that lets you unpack and repack the textures Spelunky uses for yourself.
Anyway, here's what it looks like in action
I made the sprite by drawing the thing at about 500% what the game uses and then resizing it to match the existing sprite size. Reason being that drawing at that low a resolution is really hard because it isn't straight pixel art.
I'd say all in all, from start to current state it was prolly about 5 hours spread across a week and half. About half of that was drawing the art and I'd say about 2 hours of resizing and getting the spritesheet placement right with another half hour of testing in between.
Cool thing about this custom skin stuff is that everything works fine with the game and the daily challenge so pretty much, I just play the game with it now. Also, it's super simple to install and uninstall the mods since the entire process consists of just changing out two files in the texture directory of Spelunky. Anyway, here's a link to it if anyone else wants to check it out. There's a readme in the .zip that'll explain how to install and uninstall it.