Custom Skin (downloaded/created) Issue - "That's not a skin silly"

I cannot use custom skins (downloaded/created) from Photos/Gallery, tried multiple different skins but didn't work. Skin system didn't recognise them as 'Skins' and gave the pop-up message "That's not a skin silly." Not sure if other people/players are having the same issue. Please check this out, as I like to use custom skins whilst playing the game, plus my skins help friends & other players recognise who I am.