I have picked some images from New Year 2007 (with candles showing the number) to offer free in some sites.
In 123RF, they'll be available until the end of the month. There have been some downloads, but no evidence of promoting more sales, and anyway my sales there are always slow. You can set for how long images will be available for free.
In FP, I had two sales of similar New Year 2008 images, but I can't make any judgement because sales in FP are too little. I can make them not free at any time.
I offered some to CanStockPhoto, but I don't know they will use them.
I was going to try DT too, but images were not 1 year old yet (now they are) and this is a condition for their free images.
Regards,
Adelaide