Update link preview image for podcast page

Did you change the filename when you changed the image? That's essential because the Store caches the image and would have no way of knowing it had changed. That would cause the symptoms you see and I can confirm. As it's changed when subscribing then presumably the acrtual image meets the requirements. I don't know when you made the change: it can take the Store some days to update.

I can't look at the situation unless you post the URLs of your iTunes Store page and your feed.