Paintings not working with /entitydata command; can't change Motive tag

I have been trying to use the /entitydata command with paintings, so I can change the Motive tag.

I typed in the following command:
/entitydata @e[type=Painting]

{Motive:Aztec2}

but the painting did not change. I tried it with another painting (different art), but the same thing happened, so I know that the original painting had the tag

{Motive:Aztec2}

.

Also, I am using 1.8, not 1.8.1, but the page won't let me submit with 1.8 in the box