Hi,
To upload a skin don't think it will be done quiclly i need to see if it's ot a security issue trough the player.
But in the advanced version it's plan to alllow user to choose his skin on a list already on the server.
for the logo it's quite different, it's possible using a png file by transparent on the player, it's already possible but only by the administrator.
why not to allow this for users, the idea is too relate with your another question about upload files, it's an idea that i am thinking.