I copy and pasted this info from google fyi and done a lil research , Because i was really wanted to know i figured it was the way it was but nope! I tried finding the original painting if any one has any leads lemme know.

Also you can see the cropping on Sandys head in a few frames..

I thought it was a cool detail!

That is not actually a standard painting, but rather a vintage Coca-Cola advertisement poster featuring a group of people enjoying drinks.

If you are wondering why it looks incredibly blurry and out of focus in the movie, it wasn't a creative choice by the director—it was the result of a last-minute corporate product placement war!

The Story Behind the Blur

• The Conflict: During production, scenes inside the Frosty Palace diner were filmed with massive, authentic Coca-Cola signage and advertisements prominently displayed on the walls. However, before the film hit theaters, producer Allan Carr struck a major cross-promotional product placement deal with Coke’s fierce rival, Pepsi (which is why you see a Pepsi logo in the animated opening credits).
• The Solution: When Carr realized the diner scenes were full of Coca-Cola branding, he ordered director Randal Kleiser to fix it. Re-shooting the scenes was far too expensive and time-consuming. Instead, they used optical mattes to manually blur out the offending logos frame-by-frame.
• The Giveaways: Because the technology in 1978 wasn't as seamless as modern digital editing, the blur effects "wiggle" slightly on screen. While they managed to obscure the main text on this poster, you can still clearly see the iconic red-and-white circular Coca-Cola logo script in the bottom right corner, and a red Coke cooler elsewhere in the diner was left entirely unblurred because it was too difficult to cover up.