Almost everything about art is subjective: genre, images, placement, colors, size. Of course, a lot depends on the space including ceiling height, furniture, lighting - natural and artificial. However, in most instances, art should be hung at eye level. Yes, depending on our height our eye level varies, but 5'6" from the floor to the center of the image is a good starting point. If that doesn't work for you, make adjustments that suit you. Stay away from hanging art too high. You want to be pulled into the image, not feel like it's out of your reach.
If the painting/photo/print is going above a piece of furniture, the art should probably be hung a bit lower. Again, every situation is different.
Art enhances space and enriches our lives. Enjoy it!