I can't tell if this is an image or a vector, but the edges are jagged when zooming in on the details. There is also a slight moire pattern on it but I'm not sure that alone would be the culprit.
What program did you use to make this? if its Photo Shop then its not a true vector. If its in Illustrator then maybe the inspectors don't like the transparency you used.
But then I don't know what your tech issues are since you didn't give much details.
I made it with Flame Painter. Actually it was a vector but I exported it as a jpg.
Stock doesn't typically accept B&W images.