Once I got my complete set I was able to look over all of the player cards. In doing so I notice that it isn't some of the backgrounds that got photoshopped, but all of them were.
In fact, Topps used 20 different backgrounds. They can be divided into two groups:
This is the more popular of the two, with thirteen backgrounds from the inside of a couple old-time ballparks
Seven different backgrounds were used in a park-like setting.
Grab any card from the set and you'll find one of these 21 different backgrounds.
The cards of the non-baseball players also have photoshopped backgrounds. Some use these 20 but most use different ones. I'll try to get to them some time.