I am using the themes cover feature atm. so you might want to add something like: https://github.com/Automattic/wordpress-activitypub/pull/342/files#diff-480b2542905cc428144536831a25aa56f1c4307d2667a40f719353b6ed91bf8dR253

Or you wait until v1.0.0, then you can set the image in the WordPress customizer.

I am also looking for an alternative: https://github.com/Automattic/wordpress-activitypub/tree/header-image