First I started doing the cartoons in the wall using vector ... nothing very complicated to get the impression that really was painted on the wall.
Then I put a more 'painting' effect in the corners of the wall and arange to seem that the design has always been there.
Then applied some effects to improve the look 'painted' and darken a little bit the image
Finally added the shadow to get the impression that someone has hidden looking...

That it ^ ^