I recently enabled RSS for my own blog¹ and found it very frustrating getting the images/thumbs to display properly. The reason it was frustrating is the aggressive caching by the RSS readers. I had to debug it on a bunch of different readers, then once it was finally working change the URL of my feed to force them all to refresh.

The RSS feeds are surprisingly non-standardized for the media content extensions, even a simple thumbnail.

[1] https://www.jasonthorsness.com at https://www.jasonthorsness.com/rss.xml