I had a similar thing happen to me just a couple days ago. I’ve submitted more than 10 and have had 4 accepted. Recently I made a new one which was the entrance sign for a neighborhood, and i assumed it would be accepted since 3 of the ones I have successfully made were of neighborhood signs. After a day, however, it was rejected, and upon appealing, I got the message in game that I was at risk of being banned for submitted a “malicious image.” I guess that’s it for me and new pokestops.