As a heavy stock user (I publish print and online magazines and news sites) I've often thought about why sites don't take advantage of the search log file. It would be a clue as to what people are looking for. If you posted the top three or four hundred searches that yielded no results I'm sure you would find that a lot of photographers would be happy to fill the gaps. From my perspective ther would be no better way to grow this site with quality stock that isn't repetitive.
I think that would be inspiring for photographers.
I think some people do, and we have had topics in the past addressing this issue. Thank you for the reminder.
What are some of the type of images that you regularly have difficulty finding?
Not all photographers on free sites have access to - for example - business images, or images of people which are suitable for stock. but there are some folk who could fill the gaps, and I would urge those who can, and who are interested in increasing their downloads, to at least check the recent searches (at the bottom of the home page), to see if there are subjects with few images.
One of the other problems is that people don't have enough keywords, so their photos are not seen by people. For those who struggle to think of appropriate keywords, may I suggest you go to paysites and copy the keywords from similar images? There is also Yuri Arcurs keywording tool (Google it). Keywords are the lifeblood fo the site, and the only way your photos or graphics will be seen after they leave the recently uploaded page.