Images are reviewed at 100% by humans with "capable equipment."
It is likely the post processing rejection reason you referenced is due to the black and white conversion. Generally speaking you are better off submitting a clean, sharp version of your image in color. Most designers buying stock are capable of converting to black and white if that is their preference. We have found through customer feedback they prefer to do their own conversions so the tone matches their project exactly. We will accept some black and white content but that is the exception in most cases.
What if the image was originally shot in black and white?
It is likely we will not be able to accept it. Most black and white images are rejected regardless of how they were captured. When shooting for stock, I recommend you shoot RAW to give yourself the most flexibility.