Facebook may display an alternative image from your site if the podcast or episode image you used doesn't meet their image requirements. You should use an image that's at least 200x200px. Please see the detailed image requirements for Facebook here: https://developers.facebook.com/docs/sharing/best-practices
When the podcast image is incorrectly displayed on Facebook, you can go to https://developers.facebook.com/tools/debug. Paste your episode link there, and click Debug. You will see the Link Preview on the Facebook, including the image. If the image is wrong, please click Scrape Again.
* Facebook controls how the content display on Facebook and they recommend an image in a rectangular format ( 1.91:1 image ratio, such as 600x314px) for Facebook sharing.