I’m delighted with how the Bizarro literalizes the idea of the fourth wall.
So then the problem-solvers’ question is “what are we seeing on the left side of the drawing?” And the answer should be “The adhesive side of the wallpaper”.
The contributor sending in the Macanudo points out that it’s a very old joke (probably could be found in Mutt & Jeff :) ), but this is still a very good realization of the idea.
And they pair well because the Macanudo is at the other end of sticking to the metaphorical.
P.S. Wayno prepares two versions of each Bizarro he makes, a squarish panel and a more horizontal strip format. Occasionally in his weekly blog he shows the variation, and in the week covering this cartoon it was the one getting that treatment. Here is that alternate image:
Comics I Don't Understand 
The back side of the wall paper. The pattern is on the side you see in the room, we are on the other side of the 4th wall, so we see the blank side.
This one isn’t tagged “CIDU“, but it’s also not marked as “Not a CIDU“, so perhaps it’s understandable if people aren’t sure whether it should count as a CIDU or not.
Intriguing “cartoon physics” paradox? It’s a “fourth wall” in the theatrical sense — because it isn’t there! Oh, but wait — on the left side of the picture, the wallpaper has already been hung and (as Karl points out) we are seeing the back, with the stickum already applied and then the paper. But what were they applied to? Not the insubstantial theatrical fourth wall, because that isn’t physically present.
Well I suppose it could be said to be made of glass.
I couldn’t find the “spots” that I was looking for, but here is Bud Fisher playing with his inkwell:
Has it been noted that the date of this posting was also The Fourth?
@ Dana K – Not yet: you were the first to notice; and it wasn’t a coincidence, either.
One qu9bble with the Bizarro – you generally apply paste to the paper, not the wall. Less messy.