I have not been convinced lemmy.world being the largest instance is actually a bad thing. It’s bad for federation, I suppose, but they’re all Redditeurs and I appreciate having a containment zone for them.
Most people here are from reddit or other centralized and enshittified platforms such as Twitter.
LW got recommended often and it created a snowballing effect, which is why it became the biggest lemmy instance. Unfortunately people keep doing it and LW admins refuse to close down their registrations temporarily to allow other instances to get some traffic as well. That's why some people (like me) advise people (like you) to stop recommending LW over other instances.
And LW is not a "containment" zone for former redditors.
Seems like the image_details table was added in Lemmy v0.19.6. if you are testing it with your current instance (LW), it's still using Lemmy v0.19.3. That's why.
I made a feature request for including an image hash with the image_details table but it seems like they will be adding a blurhash field instead, which is unique enough for a temporary cache that will at most probably cache ~100 images at best.
Oh and yeah, you probably should get ready to support blurhash in your client. It both acts as a small hash and a way to show blurry "previews" of the image.
Please read the book fully before starting any real project. It will save you headaches later