Marty,
Obviously, the first factor, is that the image has to be good, at least and if betterer, the betterer the chances it will sell. If the image is unique, great quality and seen by an actual buyer, looking for that image type, it will sell.
The "seen by a buyer" is kinda important too, as mentioned above. Since most buyers come from Google to FAA and not just from FAA alone, anything that you can do, to get Mr. Google's attention will help. Example: FAA only looks at the keywords in your image. Google looks at the title,keywords, description and more, so if your image is a red 1947 Ford Coca Cola Delivery Truck, then those words need to be in at least ALL 3 locations.............1947 Ford Coca Cola Delivery Truck.................do a Google search.............
Rich