Minecraft capes: how they work
What a Minecraft cape is, which accounts have one, and what you can do with a cape PNG.
Capes are not part of the skin
A cape is a separate texture from the skin, and in vanilla Minecraft it is tied to the account, not to a file you upload. Mojang hands capes out for events, migrations and cosmetics — you cannot apply an arbitrary PNG to a vanilla account.
What the PNGs here are for
The capes on 9Minecraft Capes are the textures themselves. They are useful with cape mods, on servers that serve their own cosmetics, in resource packs, and as a reference if you are drawing your own.
Sizes
The classic cape texture is 64×32. HD capes use 1024×512 with the same layout, and need a mod that supports them.