After loading the Minecraft Launcher, I clicked on the Skins tab to upload a new skin to use. I clicked on Browse to look for the skin I found on minecraftskins.com, selected that, and hit Open. (the skins are saved under my Downloads folder) Since they are the classic model I made sure Classic was selected under Player Model. I then hit Save and Upload and that is when I got the error message, "Unable to upload skin". I then repeated the process but made sure the Slim option under Player Model was selected and I came up with the same result.