If I had a site about pictures, does it make a difference if I have my keyword as pictures, or picture? Should I use both? pictures, picture, free pictures, free pictures, buy pictures, buy picture.. Please advise
Having pictures should cover both words so if for some reason you only wanted to choose pictures that would be fine, people searching picture will find you if all other factors are ideal at the time. It depends where you mean they would be used, within the page content, meta tags?
Use both, they are indexed seperately. Also use pics, images, and every other variation you can find in strategic locations throughout your site.
SE's will make a match to plural for a singular search, but exact phrase matches rank better. For example, this search has my site showing #2. But this search is #6. Notice the sites that outrank me have it plural form in title, url, and description. Hmmm...ought to fix that.
Free Pictures and Buy Pictures would have very high traffic. I would advise you to go for these keywords. Buy cheap pictures. Download Free Pictures. Free Images Free Travel Images, Free Business Professional Images.