I'm here to report a bug, but to beg you not to fix it, or to add it to the game. It's awesome! You can add custom textures to heads using commands in the game: check https://www.youtube.com/watch?v=J0ulXBtxQvo . Please, don't fix it even tought I think it's a bug or add it to the game.

Example:

/give @p skull 1 3 {SkullOwner:{Id:00000000-0000-0000-0000-000000000000,Properties:{textures:[{Value:e3RleHR1cmVzOntTS0lOOnt1cmw6Imh0dHA6Ly9pLmltZ3VyLmNvbS9nZ1hsZWhFLnBuZyJ9fX0=}]}}}

There's also a generator where you can use any texture

(Mod edit) Copied from link above:
The following command creates a head that is not attached to any minecraft account.

/give @p skull 1 3 {SkullOwner:{Id:00000000-0000-0000-0000-000000000000,Properties:{textures:[{Value:e3RleHR1cmVzOntTS0lOOnt1cmw6Imh0dHA6Ly9pLmltZ3VyLmNvbS9nZ1hsZWhFLnBuZyJ9fX0=}]}}}

Don't see how? That strange e3RleH...J9fX0= string is actually (inexplicably...) base64. When decoded, it reads:

{textures:{SKIN:{url:"http://i.imgur.com/ggXlehE.png"}}}

I should mention that this only became possible fairly recently.

EDIT: Also, each you need a different UUID for each head texture. I strongly recommend generating one instead of just using all zeros like I did up there.