Image can help for social engagement towards your business audience. Google map allows you to add images in your list which represent the product to people and how it looks. They have certain recommended practices regarded as an image ratio.
The photo should be related to the product. We would recommend avoiding posting irrelevant photos.
It is necessary for your post to be successfully published on GMB that the location is verified. After completing the listing of your location, you must have verified your business location in Google My Business and check the location's status by clicking on the Manage location or in Social Planner, in the section of Type with verified or unverified option.
Sadly it is not possible, Google My business hasn't allowed its users to post more than 10 locations. If you added more than 10 locations then you'll receive a message from the GMB "This location belongs to a chain. The Local Post API is disabled for this location".