I actually just wrote a couple pages on this yesterday for my training course. I'll paste it in here without the screenshots as I'd have to manually download and reupload every one.
Photo Spam
In some verticals it’s a common spamming practice for individuals to upload photos of their own company onto the listings of their competitors in efforts to hijack their traffic. These photos normally have their contact information on them.
How do I see what photos users have uploaded to my listing?
Go into the GMB dashboard, click the photos tab and the “by customer” label.
Is there a way to automatically track when new photos get added?
Google doesn’t currently alert business owners when new photos have been added to their listing but if you use Bright Local for tracking, they have a feature that will track changes to your GMB listing which includes photos. You can have it set to email you every time they notice a change.
What’s the best method to report an abusive photo if I see one?
The best method is to report the user who uploaded the photos. That way Google can remove the entire account (not just the one photo) which will make it a bit harder for them to add it back again.
Click on the photo in question. In the top left corner of the page it should list the username for who added the photo next to a little photo icon.
You now have the user’s profile that will look something like this:
https://www.google.com/maps/contrib/example123
Next, you can report the spammer on the Local Guides Forum using the format mentioned
here.