Nice Picture. I think the problem is the bright corner (top,right). There are some stripes in the light, maybe these are the artifacts. You may could fix it with PS or just focus on the man and the pigeons.
It looks over-sharpened to me. Also, we do accept some black and white images but it's rare. Designers prefer to add their own post processing effects including black and white conversions to match their project. You will likely find better results submitting the clean, color version of the file. Did you submit a model release with this image? If not, you will need to do so before it can be accepted regardless of the post processing or artifacts.