The pictures inside hyrule castle do not actually "mean" anything. They ere just added in as cameo apperences, or or fun. Anybody can fantasize a reason "why nintendo added such-and-such into so-and so" but nobody can really tell you if there was a real reason for doing so. In my opinion, the pictures of Mario & Friends were stuck in for fun, and for anybody daring enough to randomly look thru a window...
Also, if you shoot the pictures via sling-shot, a solder appears and hurts you somehow.
The ten things you said are true, though. I just dont believe there is any major big reason to the pictures.
