there is no one best seller. do what you do.
painters think that photographers sell better and vice versa. its not the medium but the picture itself. and how much people like it.
but keep in mind something very important --- if your a painter, then paint. unless you can shoot photos that are as good as your paintings, then don't post photos. using them as filler will only give you less sales and will water down the content. so for example, posting up a crooked dead hosta plant, will only weaken the other items. people think you need numbers, but in reality you need quality and numbers. and if you came in with only one image, be prepared to make a lot more images.
---Mike Savad
http://www.MikeSavad.com