I'm gonna run some scripts on my instance's database to look for posts that have lemm.ee pict-rs URLs and local thumbnails. Then update the url value to that of the local thumbnail_url. That'll at least fix them on my instance where I have a copy.
I won't be doing that until after lemm.ee shuts down, otherwise the original values will federate back in if there's any activity on the posts.
For comment images or ones where my instance didn't create a thumbnail, they'll just have to be broken I guess - same goes for the community icons, though I suppose I could go ahead and get local versions of those and update the communities afterward. Prob also set those to "Mods only" in the database to keep local users from trying to post there.
For user avatars, I'll probably just clear those in the database so when looking back at old content from .ee, it won't try to fetch them and timeout/404.
There were less posts to worry about then, but for community icons and user avatars, that's the same thing I did when kbin shut down.