I had one such rejection today. That's the 7th or 8th image in a series, this was the only one with this rejection.
"We appreciate the depth of keywords associated with this image, but we felt that some were not quite appropriate. We've recently changed how we tag and index images, and the good news is that images now need fewer keywords to do well in searches. The site handles translations and synonyms, so you don't have to."
Well, I keyword my images not for IS only, so I have to add synonyms. Why do they bother if in the end they make no difference? I think it's a waste of time to reject an image because of keywords when you can add them again later if you want. It only makes sense in a site such as BigStock, where you can't, or if there is any punishment for keyword spamming.
Regards,
Adelaide