I now understand why you can’t share photos from a Mastodon client when using it with #Friendica:

In Friendica, photos have permissions. Mastodon doesn’t understand this, so when it uploads the photo it doesn’t set any. The Friendica server seems to see this and decide it needs to default to the most restrictive set, i.e. private.

So the created post does not contain a shareable photo.

If you then go into your gallery and edit the photo permissions to be public, then make a bullshit edit to the post, so that it refederates, it’ll suddenly appear.