I imported your image into Lightroom and looked at it under 8:1 magnification. I saw noticeable jpeg artifacts in the greenish field in the upper right. I also so them in other areas of the background. I think this is what they are talking about. So far I have had good luck submitting by judging my images 8:1. I would have tried two things to fix the artifacts. One is eliminating all the sharpening in the image. The other is the luminance slider under Noise Reduction in the Lightshop Develop tools. Hope that helps!