It seems to me looking through these threads that StockXpert are being very selective in what they are looking for. I got rejected for the same reason, and told to look at the most poular images. Which is fair enough, and I'm not complaining.
However what would seem to make sense to me is a good definitve description of what they are looking for. Whether it be technically perfect images taken with a Hasslebland with 45mb leaf back or No animals, flowers or household items isolated on white. There seems to be a lot of people saying they won't be bothering uploading to the site. At least if they had a better idea of what to upload, and what not to bother with, it should lead to happier potential suppliers. Also it may save the reviewers a lot of time looking at images that don't stand a chance of getting accepted.