Yeah, I wonder if that would fly with the users. I just scrolled through my timeline and nearly every post has some colorful image to it. (except in Ask Lemmy and No Stupid Questions.) I'm not sure if users would accept this platform if it were mostly textual. And putting restrictions in place would certainly reduce the number of images. Scrolling through Lemmy would feel like Hackernews, not any modern social media platform. I doubt mainstream people appreciate that.

But yeah, that'd be possible. We could just close the meme communities for example. Or exclude them from individual instances to save some space there.

limit their size or the volume allowed per user

Fedi software like Misskey already did exactly that. Each user have limited "drive" which can be upgraded/customized per user. People even reuse image they already have on their drive, so it won't be any duplicate files.