Recently I uploaded a Venice collection as well. These protected elements might be small details such as license plates or names of boats, the transport company, trademarks of engines of boats,... I remember I removed these from my pics of the Canal Grande before submitting (besides the more obvious shop or other brand trademarks.
Enlarge the image at 100% pixel size and look for ANY product brand/logos, or recognizable people. Also, certain buildings are copyright protected. If you find any, the image should be submitted as Editorial. There are many sources of information on this subject on blogs or DT tutorial.
When you start putting your best images forward and they are rejected you feel grumpy and like giving up. But persist and soon you'll understand editorial thinking and standards. Don't let rejection depress you; just learn from it.