100kb is filesize limit on your instance.
Source: https://lemm.ee/post/37715

Edit:
Nevermind it seems they blocked all image uploads now.
Source: https://lemm.ee/post/5905754

You can upload it to 3rd party image hosting service and then add link to it in the URL field

Or if you want to it in the body you can do this:

![alt text](https://lemm.ee/static/fd8d452/assets/icons/icon-96x96.png)

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