Couldn't tell you specifically. If you still have the original sent email, double-check it and make sure
- the image is attached as a file or embedded in the email
- your RPoL username is clearly included in the subject line or body of the email
Those are the two big oopses that people make, and if either of those requirements isn't met, it's rejected. You can look at the list of other rules to be sure whether it may accidentally have violated any of those (such as being a duplicate), but those two are the main ones that we see.
If you've still got the first email you sent and it's something like the image was a link rather than an attachment, just fix that and send it again. We can roll it into the next batch, no problem.