Not letting me put my custom Minecraft skin

So I've been starting to play Minecraft again and when I downloaded it again it won't let me put my custom Minecraft skin anymore. I don't know why but I've been using that same skin(s) for the past 2 or 3 years and now it won't work and it is a real Minecraft skin because I've been using it for the past years so I'm not sure why it's not letting me now but if you could please fix this bug.