but it might even be possible to intercept images to check that this exists in the meta data before Lemmy wipes it....
This would be impossible without directly modifying Lemmy itself.
There could be a few trusted folk out there turning their servers into S3 nodes (?) via minio or something like that, which Lemmy's pict-rs could use as storage. The main Lemmy backend process serves out the images anyway (I have no real idea why pict-rs is that separate tbh), so throwing a CDN in front of that (filtered to images and video) should be enough to get some reliability on serving images.
Of course this is only the technical stuff. The legal parts are a completely different issue altogether
I have proposed something of this nature to the Lemmy developers and am waiting on feedback from them.