or compression rate, it is best to leave on maximum - large file, so in Photoshop that setting would be 12. This should then reduce jpeg artifacts and other potential problems. So, to be clear, these were uploaded as illustrations, right, and you submitted them as JPEG files, right. The only difference between these two is the background. Blue being accepted - as you stated and white background rejected due to 'artifacts', right? So, it seems that with a white or grey background, this somehow made a difference in reviewing. Which of course does not make a lot of sense. Perhaps, with the white background it created an optical illusion - giving an appearance of being blurred, or having artifacts. Whereas with the blue background, this didn't occur. So, maybe, submit again. Start with a new file, and save as a large file (maximum) and see what happens. Although, I guess you do this anyway! Personally, I can't see any difference between the two files - apart from one white background and one blue.
... View more