Hi all!
The answer to this is that background tasks cannot return media objects directly (as @danbolinson theorised) - you’ll have to store them in Data Tables and retrieve them from there.
The reasoning behind this is that the return values from background tasks can be accessed for a long time, and therefore it would be possible for large media objects to exhaust your data storage quota in a way that’s not very visible to you, or easy to fix. I hope this helps to clarify the situation here!