Cannot upload custom skin

When downloading an image of a Minecraft skin it doesn't let me, it comes out as if it is an error, it says that it is not a skin of Minecraft cork oak and I have tried to download it on other pages or applications, the same thing comes up and I have previously used skins and I don't know why Now it comes out like this, it's been several months and I've updated it to see if it was some error and it doesn't come out the same. I hope you can help me please and thank you