There are 36 pictures in the 6x6 Votto grid. The challenge is to figure out what 12 of those images have in common. Apart from merely looking at the images, certain constraints within the puzzle aid the solver in arriving at the solution. Each row and column has exactly two images which share the common trait. Here is a screenshot of a Votto puzzle:

In the left-hand column, there are hint features as well as a solution link. Each hint merely highlights a square. Can you figure out what 12 of these images have in common? Remember, there are only two images per row and column.