The painting side look glitchy, same kind of rendering as when two painting are overlaping.
Moreover, when using a texture pack with painting wich doesn't fill the "square" in the "kz.png" file (by the way, it should be renamed), you can still see the side even inside the frame.
I think adding a file dedicated to the painting side in the "art" directory should solve the problem
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.