That's right -- the first photo you post in any of those categories receives an acorn, but not subsequent photos. However, when you get into the Plant Database screen, up in the upper left corner it shows the total number of plants in the database, followed by (view more stats). If you click on the "view more stats" it gives different statistics, including the total number of pictures by person. And please note that both Paul and I are in the top ten!
And, of course, Calif Sue is so far ahead of everyone else that no one has a chance of catching her -- but it's sort of a goal to see how many total pictures you can get. So we just keep posting away!