Google tell reviewers just how many times their photos have been viewed. Here are some examples:
...with over three thousand views in three months.
A car repair service:
An art gallery:
Here is an example of the weekly statistics Google send to their Local Guides*:
Encourage your reviewers to post images. They get seen - and aren't they sometimes 'worth a thousand words'?
N.B. *Be aware of the new Google points system - Local Guides are motivated by it.