Showing only reviews with photos
The short answer
Two separate settings control this. Show Images on the Review tab decides whether attached photos are displayed at all, and it is off by default. Show Reviews Only with Photos in Filters decides whether reviews without photos are hidden.
Turning on the first does not do the second.
Showing the photos
Review tab, switch on Show Images. Photos attached by reviewers then appear in the review cards. This is off by default, which is why most people’s first assumption is that the widget does not support photos.
Showing only reviews that have photos
Settings tab, Filters, switch on Show Reviews Only with Photos. Everything without an image is hidden. Combine the two and you get a photo wall, which works well for restaurants, salons and anything visual.
Sorting by photos instead
If you do not want to hide the text-only reviews, set sorting to Reviews with photos first. The visual ones lead and the rest follow.