Quality is highly subjective. Some of what I think are my best images sell very little but those that I almost didn't upload sell well. I see a few people with small portfolios packed with images that sell well but I think that would be hard for me to do. I just upload everything that I think might sell and let the buyers decide. The best images rise to the top in my portfolio and some of them are a real surprise.
Deciding what to sell on micro and macro sites is tricky as well. A high quality image on a macro site might never sell or only sell a few times and make less than it would on the microstock sites.