I never set a free image in DT, but as far as I know, you can not specify a period for this (unlike 123RF) or withdraw it from the free section (unlike FP).
If I remember it right, approved images have to be in the site for over a year to be allowed into the free section. I tried that for a New Year 2007 image but it was too early, then I didn't try again later.
Regards,
Adelaide