The last significant change I have seen was in January of 07, when I basically doubled my daily download average from 4.06 to 8.77. By February I'd tripled it to 13.00 downloads a day. That is factoring in decent, even great (for me) weekdays against slow, sometimes only 1 or 2, weekend days. After February of 2007, I have averaged 20-25 uploads each month and my sales have stayed relatively consistent with lower numbers in January, May and June of this year. My overall average for those 18 months is 11.60 dls/day. So far, August is averaging 10.66/day, which is pretty consistent.
My point being that for me, uploading 20-25 images per month maintains (but only maintains) consistent sales. If I uploaded more, I would expect an increase. If I uploaded less, I would expect my daily average to begin to drop on a consistent basis.