You should upgrade to Podlove Publisher Version 2.2.0 or higher. With this version the use of Wordpress’ image caching was introduced. Since then smaller versions of your images are rendered on the server and delivered to the browser. That saves bandwith and time for transmitting.
After upgrading you should use the new syntax for handling images. In your case the new syntax should be something like this:
The caching API only returns URLs to the resized images if they already exist on disk. If they are not there, it returns the original URL and creates an internal job to cache and resize the image.
If you use some sort of page cache there is a scenario where the HTML of the img with original file URLs gets cached. Another reason could be that something prevents the image cache from working correctly, for example missing file permissions.
Pleaser verify:
That it’s no other (page) cache interfering
That the image cache is generally working. Cached files should be in wp-content/cache/podlove/