the game is not accepting my skins

Since the last update the app had, it won't let me put my custom skins despite having used them in the past. I just want to specify that the format of the aspects is correct, since, again, I have used them before.

The thing is that despite the fact that I have tried to put the skins several times, and I have even reinstalled them, it does not let me import them since I get a sign that says "this is not a Minecraft skin". I don't know if the format for importing skins has changed, but I'm worried that I won't be able to get back the ones I've used for years, since they were the ones I installed from a safe and trustworthy site.