Please check the below:
- jquery - Access Tableau thumbnails for views - Stack Overflow
- Thumbnails in Tableau Public Profile
- Workbook Thumbnail Images Missing from Home Page | Tableau Software
- Workbook With User Filters are Displayed with a Generic Image When Published to Tableau Server | Tableau Software
- https://www.reddit.com/r/tableau/comments/7pz7cw/tableau_public_thumbnails_not_s howing_up/
hope it helps you.
This is a cache issue on the client side. I thought I had cleared the cache in Chrome, but I had failed to check "Clear cookies and other site data". Checking this box seemed to have corrected the problem.