I've signed up with several microstock sites and all I can say is I don't think the same two photos have been accepted on more than one site. They all have different ideas of what works.
I would recommend trying to get on to istockphoto.com, they seem to be less capricious than the other sites as to what they will accept.
123 Royalty Free - Stock Image and Photo Subscription will also accept pretty much anything (which won't push you to get better, but may get you a sale or two).
I'd posted a link to this article before, but it serves repeating I think...
Articles - Are You Taking Snapshots? | iStockphoto.com
edit-- Just a quick note on your pic IMHO... 1. Needs to be brighter, closer and loose the dead leaves.. only show the colourful ones. 2. I reckon this one would be accepted on istock (provided there are no technical issues like noise etc in the large file). 3. This concept will sell, but get lower and make the apple bigger in the frame. Here is a similar one as an example, with over 100 downloads
