This is a theme that you hear not only in photography but in stock photography as well. I can attest that this is the case if you put forth the effort.
Things were rocky when I started about two years ago. My first month were all rejections, but I kept at it and here is the results two years later.
Q3 2011 Acceptance Ratio 31.7%
Q3 2012 Acceptance Ratio 64.5%
Q2 2013 Acceptance Ration 91.0 %
743 Images on line.
I still need improvement, I am still learning all the time. Improve my lighting, improve my composition, and most of all pursuing creating images that convey concepts and ideas better. Still, the adage holds true, keep shooting, keep improving.