There is some information here, look under PHOTOGRAPHY AND DESIGN TUTORIALS: http://www.dreamstime.com/stockutilities
One thing to learn is the rejection messages are canned. Reviewers have some kind of list to choose from and will select the appropriate message. At the same time, it's not very helpful.
It could be you're using a point-and-shoot and zoomed in a bit with the optical lens. You could have exposure problems which will create noise in the darker areas. You might have cranked up the color saturation too much. Or the contrast too much.
You may have not done anything to the image because you don't have photo editing software. It's imperative to have good photo editing software. And then invest the time to learn the software.
You may need to calibrate your monitor so you're looking at true colors and not skewed.
Write to support and ask the Reviewer exactly where in the image they are seeing issues. That will help to identify what to look for.
Welcome to the world of commercial and professional photography. I don't have access to the Dreamstime database but I would estimate at least 75% who join here quit and give up.
Nikon D800, D100, Canon G15