I have written a jlab4 extension, and when it shows up in the extension manager it comes with an icon that I have not specified explicitly, but that could have been found as the avatar I use on github
That thread is not relevant for JupyterLab 4 which has a new extension manager.
No way currently I am afraid (other than publishing under an organization). We coud potentially agree on a metadata field e.g. keyword which would mean “do not show my image as logo”, but there is no dedicated URL field we could use AFAIK.
Maybe we could just check for a picture in a prespecified location in the GitHub repository? This would be a good question to raise during weekly dev meeting.