I think part of the benefit of offering a free image is found if you have other images in your portfolio that are keyworded with close matches. I have one free photo and when you click on it you see about 15 of my photos for sale. In your case I think you have too small a portfolio to get that benefit - you don't get any free advertising out of it. That said - I really have no way to tell if it helped my sales. I have also noticed that google searches turn up the free photos, which may draw people to the site, as Tara says.
yes, i also noticed that donating 4 free images hasnt given me any additional sales, but i dont mind as i also purchase photos on the site and do utilize the free images often in my work. i'd say that as a buyer the free images help keep me on the site.
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