It could be a good idea to add support for mineskin textures by having a mineskin-<id> format.
So f.e. mineskin-1 would automatically apply the skin of https://mineskin.org/1
Tho, with that said, it seems like MineSkin has altered their ID system to use hashes instead of nummerical IDs (Tho from my testing are nummerical IDs still working for the most part)