Banner artwork
Personally I'm not very fond of the copy & paste banners. Simply, because it takes the fun out of having to travel to get an epic picture on my scanner. For me that's what the game is about, exploring new areas through banners. On the other hand I understand that people use them for simple (Second Sunday) banners. The good thing about these banners is that the artwork is shared publicly and there is no reason why you cannot (re-)use it. Examples can be found in the #MissionProject Telegram chat.
When I search for artwork for my banners, I normally try to get away with using copyright images for banners. Why? Obviously because my digital art skills are not that good and getting a beautiful picture past the monkeys is good sport.
In many discussions I read that you shouldn't use a copyright image, because of the risk of having it rejected, but I think you should. So why not try. If the banner passes fierce eyes (AI) of the monkeys, you'll be able to get that beautiful picture online as a banner.
On failure, with the comment that "the image isn't yours", you can try using filters or alter the picture somewhat. Even adding noise to the part of the mission picture that isn't shown in the scanner sometimes helps.
Note: Common understanding is not to copy someone's banners without consent. It is fairly easy to grab mission pictures from either Ingress Intel or Bannergress, but remember that some agents create their mission images themselves or spend serious time in assembling pictures from various sources. Better ask (via Ingress Chat, Campfire or Telegram) whether it's okay to re-use those images, instead of just copying them.