In my recent Pixels Away blog post I reviewed earnings and best selling pictures during my 9 months with iStock:
http://www.pixelsaway.com/C911796005/E20080909084854/index.html

Here is a graph showing number of files in my portfolio, downloads and earnings:




I am uploading new pictures within iStock weekly limit with the acceptance rate about 70%, but I see a clear plateau in the number of downloads starting in May. September doesn't look better than August.

I wonder if it is a general trend or I really need to think about some changes in my portfolio.

I am going to do a similar analysis for SS and DT.

Perhaps your plateau is due to your advertising your most successful images, which people have since duplicated and are selling en masse?

I'm in microstock business for two and half years now and I've also spent quite a lot of time trying to make nice graphics to analyse my sales.

My conclusion now is that there are too many factors which do impact sales and it is quite useless to try to know the impact of a single or a few factors.

Put your efforts in making more and better pictures: this is the key to success 

Nothing, but advertising the fact doesn't help any.