I opened the editor view in the debugger, and I noticed the image wasn’t requested in the Network tab. When I try to open the image address in a new URL (address was http://127.0.0.1:40459/paste-f8fb365a79c18feb2aa09f517d1637c913d39dc3.png, and paste-f8...png does exist in the collection.media folder), I see the following error:
send_file() got an unexpected keyword argument 'max_age'
It looked like a Flask problem of some kind - I looked up the version of Flask currently required by anki and upgraded. Images display now!
For others coming across this issue, upgrading Flask locally to whatever version is listed here should work!