Most catalogs have photos next to the description of each tomato. I've seen watercolors used...
... for general categories
http://www.southernexposure.com/vegetables-c-3.html
...on posters
http://shop.wildboarfarms.com/Wild-B...OSTER18x24.htm
...on seed packets
http://www.groworganic.com/seeds/veg...ato-seeds.html
Is one of these what you were thinking of?