I agree with Jenn, Michael. There is no exact formula as to what to shoot that will sell. Of course, as you can see from new uploads and downloads, business and lifestyle concepts do very well. However, I have photos of my daughter and grandsons that sell, too. And, the shots I think are not going to sell usually do, while others I have high hopes for sit....but they may sell in a few months. Again, it all depends on what the designers and/or photo buyers are looking for at the moment and if your photos fit the criteria.
My advice is to take the best images possible and concentrate on good exposures and compositions. Try to add people to travel or landscape shots. The don't have to be facing the camera or even near the camera, but people in a landscape add a different perspective. If you can recognize faces, you will need a model release. And, try to shoot subjects that others may pass by. Many of the photos I take are not shots I upload to DT because they are nature photos, and there is a lot of them on all websites. Those go to my own website for display there. Even though I may take an excellent shot of a rose, DT may reject it because they already have many of those shots. To me, that is the frustrating part....have a shot you feel may be better than those already on DT, but getting rejected because there are so many of them on DT. I recently uploaded several images of flowers and spring nature subjects. Most of them were not of the quality I usually upload, but I was doing an experiment for me. I wanted to see which images DT would reject and accept. Most of them were taken handheld with an IS lens, but I could see the compositions were not as exact as when I use my tripod. Two were very good shots of a spring flower where I used my tripod, an extension tube and carefully paid attention to composition and DOF. Well, those two shots were rejected because they already had too many of them. Two of my shots were accepted....and those were the ones I felt were not my best work, or what I'm capable of producing.
Don't forget that looking at photos and choosing good images is very subjective and depends on the person looking at it and making the decision. I always make sure my images are well lit, exposed correctly, have no noise or other artifacts before I upload. I try to keep my standards high when submitting photos. If you see a small defect or flaw, others will see it too. I will not upload those shots.
Good luck and keep shooting. Your portfolio is off to a good start.
Rebel, Canon 17-85mm, Canon 18-55mm, Canon 55-200mm, Canon 75-300mm; ...