I have seen that only when an image has been in DT for three years and hasn't got any sales can be submitted to the free section. But before you try to give them for free leave them for a while to see if you can get money out of them.
Please address such questions on the message board or to contact directly. The blog section is rarely moderated so the admins cannot answer your questions. If your refused files were not accepted in the free section, even if you checked this option, then they were not accepted as free. These images still need to meet certain criteria to be accepted as free.