8x10 Promo Photo

8x10 photos became popular at some point in the 1920's-- and continues today-- as the size most popular to print promotional photos.

Another way to look at is that 4x6 is the standard 35mm camera print size, and it mattes to 5x7 with a 1/2" border around the edges. A 5x7 print mattes to 8x10 with a 1-1/2" border. An 8x10 print has the same aspect ratio as a 5x7, and is a 2x enlargement of a piece of 4x5 sheet film.