If there are many photos on marker, (first please show pointer on image hover, so visitors can see there is a link to bigger image) you shoul give us arrows to move next or previous on popup
also, under photo in Infowindow there should be “warning” like “browse photos” .. at the moment visitors cannot know that there are more photos until they start to carousel
I will kindly list these on our development schedule for our lead developer to take a look at. If approved, we will kindly include this in a future update of WP Google Maps.