Quote from Denton Harryman on February 27, 2022, 1:09 pm

Answer: Research has found that the reason is most often caused by changes Google Blogger has made relative to how photos are stored.   Fortunately, we have customized the WP RSS Aggregator plugin and the problem appears to be resolved.

As part of our efforts to solve the blank thumbnails we have also improved Search Engine Optimization (SEO).  Specifically, the thumbnail file names now are composed of your site name plus the post name.   Prior to this improvement the thumbnail file names matched the file names used by blogger which was unfriendly Globally Unique Identifier (GUID).

Examples file names

• Before change: AVvXsEiEA5Kxdt4Ru-NT9O...
• After change:    Phoenix Daily Photo – Teeny, Tiny Blooms.jpeg

Following are articles on the subject of Bloggers image extension (and file name) changes.

• New Blogger image extension change, will it effect Image SEO results? - Blogger Community (google.com)
• Too Clever By Half: Photos in Blogger changing under the hood (too-clever-by-half.blogspot.com)