It always works that way, yes. The more I upload (in working days (weekends are a different story)), the more sales I get. It is obviously a matter of quality as well (not really quality, but "buyer suppositions based on thumbnail taste"), but mainly yes, I claim that Shutterstock works that way.
p.s.: It's certainly because of the 25 images per day subscription quota. Buyers check new images under the liked categories and go haywire "downloading everything that breathes"
![Wink ;)](http://forum.dolgachov.com/Smileys/default/wink.gif)
And consequently they check the portfolios as well. If you don't upload (or don't upload that much), you don't get downloads.
p.p.s.: Other sites (most of them) work completely differently. It is my belief that a big chunk of downloads (again relatively) on Shutterstock happens without targeted search engine inputs.