For over a year I am a contributor to DT and after that year, I did put some pictures in the free section that were not even looked at. To my surprise these images are now the best downloaded ones of my portfolio.
There are more than 97.000 photographers active on DT alone. If every one of them has 10 pictures in the free section, well you can do the mathematics. That is 970.000 free photo's! I am sure there will be a usefull one for every designer. I have 4 pictures in the free section, but I won't put anymore into it!
This free photo for instant is downloaded more than 20 times in only two weeks. Some people say that the sales will go up if you have pictures in the free section, but I have my doubts.
Royalty free, nine views, zero downloads....
Have a nice day!