Eliot has it. Let me try too:) There is always a dozen correct ways to get something like this done. Sometimes it is better to not put to much in a sentence, and let information be separate. Very TRUE. But in this case, we can fit it all in. We are assuming the picture is not actually drawn on the wall. It is hanging - or taped - in the wall... The drawing hanging on the wall is of a boy drinking a Coke.

There's a picture on the wall of a boy drinking coke.

There is a picture of a boy is drinking coke on the wall.