To add artwork to an episode, you'll first need go through the upload process. From uploading your audio file, to giving it a name, description, publish date, everything you would do normally. When you're done uploading, hit save and your upload will start processing.
At this point, your processed episode will have your show's default artwork attached to it. This is how it has always worked, so there's no need to do anything else if you don't want to add custom artwork to your episode.
Edit an episode
Once your episode is done processing, you can now add the artwork. Click on the Pencil icon to edit the episode. Note: You can also click on the episode title and then the Edit button that's on the episode detail page.
Click the Choose file button in the Episode Artwork - Optional section.
For best quality, your artwork should be 1400x1400px, but don't worry too much about it. This feature is able to scale your artwork to the right size and color if it's needed.
Once you hit "Save episode info," Buzzsprout will immediately attach the artwork to the audio file and publish it in your Buzzsprout public page. Since it's attached to your file via it's iD3 tag, if anyone downloads your mp3, they'll see that artwork instead of your show's default art. It will also update in Apple Podcasts in 2-6 hours.