The maximum file size limit for images hosted directly on lemm.ee servers is always visible in the sidebar of our front page. The limit stands at 500kb per image currently.
Additionally, It’s also possible to post images larger than 500kb, they just have to be hosted externally. The markdown for this is ![alt text](image url)
FWIW I know some users often check their local announcement/support communities through other instances during downtime, so it can actually be quite nice to have such communities federated to other instances.