I find it pretty common given a list of foods that I like, the one with a good picture is the one I'll most likely order. Having more pictures may or may not benefit the restaurant though...

Use of a few pictures now could lead customers to buy a higher profit item, or an item that is much easier to keep in stock in large amounts.

Use of pictures for all items means that the customer will likely expect the food to the table looks like the item in the picture. This could cause some issues with different cooks or item substitutions.