What does Renoir have to do with it? Is the hiker a city guy seeing a city scene (deli with hanging salamis) in a painting that isn’t really that? Or that actually <i>is</i> the painting, rather than the pastoral landscape we expect?
Blame it on the Steve Martin
Comics I Don't Understand 
The artist’s “side glance” indicates that he is insulted, so the painting is a probably landscape, which the hiker had misidentified as a salami. For the humor to work, the misidentification has to be extreme. Confusing “Everest” with “Matterhorn” would be understandable, and therefore not funny. This gag works because the word “salami” is completely unexpected, and yes, it also reeks of Steve Martin.
P.S. The signature on the back is part of the joke: even famous artists had to deal with idiotic patrons. Bliss has produced a very accurate rendition of Renoir in his later years:
I think the point is, whatever the guy IS painting, it AIN’T a salami.
P.P.S. Following onto that amazing tree in yesterday’s Sunday Funnies, one detail that bothers me about this drawing is the way that the artist’s easel is leaning. The perspective makes it look as if it were pointed toward the mountain peak in the background, rather than facing directly toward the artist.
He may not know art, but he knows what he likes.
He may not know art, but he knows when he’s hungry.