Coming back: I feel a boost in sales after donating images, a small one, but noticable, but it feels so strange to give away images for free. Every time I do that, I get hundreds and hundreds of free downloads and only a few real downloads (those particular days). It helps, though, but feels bad for the ego;/
p.s.: I'd suggest to 123RF to lower the resolution of the web format (this is the one that becomes free to download after donation), so people downloading free images will be pushed to actually pay bigger images more often. The "web format" is 1024px wide and that's imho too much. Donating such an image makes it suitable for a too wide web usage. And since the buyer base is predominantly web graphic designers, we know what this results in: fewer "real" payed downloads.