Boise Ed submitted this “Carpe Diem” panel a while back, commenting that “The Dachshund appears to be the artist, with the same picture in two different shades of brown tree bark and four different shades of green grass. Perhaps this is obvious to those who would spend big money on a canvas with only a plain circle or square, but not to me.“
Comics I Don't Understand 
It is meant to be four different trees, note the different imperfections in the bark. And trees have a fascination for dogs to pee on, examine, and apparently to paint. This being a short dog they have a limited view of the tree. Going with the dog theme, the paintings are probably scratch and sniff.
I thought the two Mutts one of whom was the artist who painted the canine urinals was a reference to Marcel Duchamp
The joke is how short the dachshund is.
I agree, the joke is in how short the dachshund is, so that his view of the world is mostly ground level.
Also, they probably also have a smell component that only a dog patron can appreciate.
@ Ian (2) – I had no idea how to find the name of the relevant artist, but that’s exactly the idea behind the headline at the top of this post.
Ian Christian is making an additional joke, in that when Marcel Duchamp submitted the urinal as a sculpture under the title Fountain, it was signed “R. Mutt 1917”.
It’s not hard to search on “Artist who exhibited a urinal as a sculpture”.