After selecting the skin I want there should be the option to select between the Steve model or the Alex model but they show up as black.
Ex.

I can click on one of the image and the skin shows up as just black. After clicking on a world though it says that the skin is invalid.
Ex.

Clarification: At the beginning everything is normal and the model shows up fine. After clicking on the custom button and choosing the skin I want do things go wrong.