The poster image field has nothing to do with episode-level artwork.
Most apps that use episode-level artwork will pull it from the ID3 tags of your media files. Are you putting your episode-level artwork in each of your show’s ID3 tags?
Some apps will pull episode-level art from your podcast feed, if you have the feed set to use episode-level art. To do that, you need to check the box next “iTunes Episode Image Field” in the Website tab of PowerPress Advanced Mode settings. Then, you must set the artwork URL for each episode inside each episode’s Podcast Episode box of your WordPress posts.
I don’t see any option to modify or add to a posts ID3 tags.
It’s best to add ID3 tags to your media files before uploading them. You can do this with the free version of Apple’s iTunes desktop software. Some DAWs allow you to add ID3 tags on export. I’m a fan of a program called ID3 Editor by PA-software. ID3 tags aren’t something you do within WordPress. Instead, you apply them to the media file before you upload it for distribution.
Are you talking about putting the hosted image link from WordPress in the ID3 tags in iTunes before uploading the MP3 file?
No. You’ll need to put the image file into the ID3 tags of your media file. This page has some information on how to do it:
https://www.thepodcasthost.com/websites-hosting/adding-cover-art-id3-tags-episode-itunes/
