[BttF 1] Doc had 4 pictures on his mantle for a practical reason

In Back to the Future, there were pictures on Doc's mantle of Isaac Newton, Benjamin Franklin, Thomas Edison, and Albert Einstein.

Doc had been thinking about time travel for years prior to 1955. He knew that if he were to ever invent a time machine, he would be tempted to go back and meet these 4 people. He also realized that doing so would carry a risk of doing something which changed those people. These changes would be reflected in the pictures. Remember, as soon as Doc saw the picture of Marty's siblings changing, he knew why. This is because Doc had already been thinking about how time travel would work. The phenomenon of “perceptible picture-changing” was already in his mind.

So Doc keeps those 4 pictures on the mantle as a track to see if his future self had invented time travel, gone back to meet them and done something to change them. They need to be placed in a very visible spot so he can easily keep an eye on them.