Paintings breakable in Adventure mode

While creating a custom map, I tested out whether I was able to break picture frames. After switching to adventure mode, I was able to hit the frame causing it to break and drop. I am not sure if this is actually a bug or is intended. However, this action is bothersome as it makes it impossible to create hidden doorways behind pictures as the player only needs to hit the pictures to see what is behind them. The command /entitydata is also not available (yet?) in MCPE/Win10, so

{invulnerable:1}

cannot be used.