You can set a separate image for each episode of a private or public feed hosted at Hello Audio.
Inside the feed, click into an episode then hover and click on the cover art shown for the feed. Click "Update Image" and select the image you'd like for this episode.
This brief video shows an example of setting the image for an episode, and how it appears inside a private feed on Spotify.
The episode is updated instantly inside the feed that's delivered by Hello Audio. Podcast apps may take a few minutes to reflect the change. In the Spotify example above, it took about 3 minutes to update. Not all apps support individual episode artwork. Some may support the feature on some devices, like iOS, but not on desktop or in browser, for example.
In Hello Audio, the individual episode artwork is part of the RSS feed of the podcast. Most apps that support episode artwork look to the feed in this way to see the image. Overcast is an example of looking at the mp3 file's ID3 tags, which is not how Hello Audio delivers the artwork. You're welcome to encode the media file with that image before uploading if you'd like to use ID3-base episode artwork in an mp3 file.