For a few months before I found my feet in microstock I was clicking on the box "if not accepted submit for the free section".
Initailly I got a few noticeable payments 10 and 20c then they quickly dropped to 0.002c
I was looking at the left of the screen last month and I noticed my free downkloads had crept up to 1,000 so I bit the bullet and went through my portfolio to find these free images. Also free Dls are included in your DLs/day stats (thanks really useful)
I think there were about 40-50 in total some were borderline crap but there were a few which weren't pretty but got the job done. One in particular is (my current third best seller on SS and had 7 DLs at DT) had 30 free DLs at FT.
So beware, I think now there is virtually no point submitting those free images especially as there are sites which have different ideas what is stock where someone will pay cash. A "one off" free image that appears on the frontpage is another thing entirely.