The eight letters is easy. People grew up using MS OS which only allowed the eight.three naming conventions. Seems my whole collection of files, photos, articles and everything, before Windows 2000, is only eight letter file names.

My keywords are not so good for pushing up views, but they are specific. Now, some people want to put down banana when you are looking at an apple, because it's fruit, and they get the people to see their images. But it's counter-productive and a waste of time for buyers to see images that aren't what they are looking for.
Interesting about the plurals. I'd think the search engines would be more intuitive and when someone types in "girl" it would find the word, partial occurrences and therefore "girls".
What was it someone wrote, that very few people search for the word "isolated"? Makes sense that they wouldn't because it would be half the photos on a website.
Good luck with the research and the software.