After some of my portfolio images became one year old I decided to try this option. Some people wrote on message board this can be useful for sales, others objected, but you never know untill you see it with your own eyes.
I was incouraged and soon donated two more images:
. But these donations were not followed by any n/a sales. All images sold later were found by key-words in commercial section and had nothing to do with the ones which appeared on clicking the free ones. The first donated picture since then also didn't bring me any extras.
After this experiment my assumption is:
1. Donating free images doesn't make you "rich fast". It may bring you sales, but with no guarantee.
2. Still this option can be used as an easy opportunity to advertise.
3. Better treat it as long-term investment.
4. Donating images for free helps you analyze the needs of potential buyers much faster than by just selling. For free people upload easily and a lot, and you immediately find out which of your free pictures is more popular. Probably later you decide to enlarge your portfolio by working more on a popular concept.
5. There is always a chance that one of your free pictures "shoots" by accident and becomes well known by creeping all over. I personally know photographer whose career began to develop after one of his pictures was stolen from personal page and widely used internet (absolutely illegally), then appeared on bars and clubs' flyers (a shameless copyright violation), but after this violation people began to search this photographer to order him paid photosessions. With free images you help people to make you famous without breaking the law ;)
In life anything can happen and most of things happening to us are the results of many small steps, one of which can also be donating pictures for free. Why not?