I have seen many DT newcomers with questions and seeking advice. Not to say that I am the best person to bring up this topic, with my 51% acceptance ratio, but DT allows you to resubmit a photo after being rejected for a reason. There are multiple DT reviewers and they definitely are there to help you build your portfolio and succeed.
The attach picture was previously rejected. Instead of taking it to heart I resubmitted it. Now with 18 downloads and $60 income, though it is not much of a photo, just wood panels, it is my "glowing" accomplishment, and a reminder not to give up.
I know this is a controversial and somewhat frown upon subject but, the one difficult thing with DT rejections is if you have "too many" of the same subject. Pretty much it seems like you can only upload ONE picture of that subject, the "regulations" with this now seems a bit more strict. I just recently uploaded for review three of the same pics, one was chosen and the two others were rejected for the "too many of the same subject".
I believe,however, that if you actually find a niche where the "picture bank" is low, you should be allowed to saturate that market with your pics. For example, Bed Bugs are a huge problem throughout the world. I was lucky enough to have a microscope that could take pictures, and a nice collection of live bed bugs. DT was limited (like 1 page worth) with Bed Bug pictures. So, I obviously wanted to take advantage. I wanted to take pictures of all life stages, on white backgrounds, on pillows, on blankets, everything that would sell sell sell! but, I got rejected due to "too many of the same subject".
I read a blog once, too, that a client needed pictures of making a sandwich. Another needed for multiple images of the same subject. The photographer wrote a blog to say his pictures of the step-by-step guide got rejected for that same "too many" of the same subject.
I wonder, while filling out the info for a resubmission, if you can write to the reviewer that you believe the database for this subject is low and it would be helpful to add more pictures of this subject. I'm going to keep trying, take DT reviewers advice, and resubmit!