This can be easily implemented client-side, like how third-party Reddit clients have been doing for years, by uploading to the likes of Imgur. Shift the responsibility away from the Lemmy server and onto dedicated image hosts, who have the storage to spare, plus manpower/policies to deal with illegal content.
Or just disable images completely. We got by on Reddit without any built-in image hosting functionality for over a decade, so Lemmy should be fine without it as well - especially considering that we don’t really have many image-heavy communities, besides the NSFW instances. I mean, storage costs money you know, and with Lemmy being run by volunteers, it makes even more sense to get rid of image hosting to save costs.
So do Desktop tools like Flameshot, which can directly upload to image hosts and copy the URL to the clipboard which makes it easy to share images, and there also exists third-party Desktop web-clients such as Photon, which could be updated with that functionality as well. But with Lemmy itself being open source, it wouldn’t take much effort to modify the code to use a third-party image host.
There are plenty of hosts which do allow hotlinking though, like imgbb.com
Not a big loss, IMO. Lemmy isn’t an image hosting nor an image-centric site, it’s a text-heavy forum at first instance, and anyone posting images are encouraged to provide text alts for the benefit of blind users, so images not persisting isn’t a big deal.
If image persistence is really that important, there are other services which are better suited for that, such as Pixelfed. But in the first place, I wouldn’t rely on some random Lemmy server, which is vulnerable to DDoS and other attacks and could go down at any time (also why the importance on decentralization - no single instance is infallible). I mean, when there’s no guarantee that a Lemmy instance will even be there tomorrow, is there really a need to worry about image persistence?