It must be the analyst in me, because I hear lots of stories, but simply do not believe them. So for the past few years I have been keeping a track of around 1000 photographers on Dreamstime, recording their monthly sales, and getting information back from lots of people about the return per image they make.
Now let me just say, this is an inexact science, because few people will actually tell you how much they make. And, this is based on the fact that it is rare for you to get an EL sale.
And, to add a measure of difficulty, it seems you really cannot use this theory unless you have been on the site for at least 12 months, and have at least 100 images in your portfolio.
But here it is. On average (based on my figures) you will make 17 cents per image you have online per month.
DISCLAIMER: And a big one at that. If you are fussy, and do not load series after series as many of the bigger portfolio owners do, you will make much much more.
But this is based on the little information I can get.