Flickr user Mr Biscuit took these photos and wrote:
Personal pics from the set of Back of the Future taken while I was a security guard at Universal Studios. Hey, I heard Steven Spielberg snuck onto the lot in a security uniform, so I took it to the next level and got a job there. ...
They built the 1955 set first, then when I came back just two weeks later, they had rebuilt the same set for 1985, paving over the courthouse square park and completely remodeling all the storefronts and signage. Amazing! And they did this TWICE, since they originally shot the entire movie with Eric Stoltz, who was later replaced by Fox.
Link [Flickr] - Thanks Geith!