As someone who knows nothing about the protocol, but knows HTML, I thought I could use any URL for the <img> tag. It didn't occur to me that only Matrix urls would be allowed, which tripped me up for a good hour.
I think the text should somehow say that only Matrix urls are accepted
The input box for the community description initially looks like this:

The "You can even use 'img' tags" sentence should probably mention that only mxc URLs are allowed.
This seems to trip people up frequently.
As an alternative, it could make use of https://github.com/matrix-org/matrix-doc/issues/1270 (or similar) and automatically upload the image.
How do I get a martix URL and how to use it?
The easiest way to get one is to upload the image to a room somewhere, click on the "..." button that appears when you hover-over the image, click on "View Source", and look for the "mxc://..." URL. Once you have that, then you can use it in the img tag as "\
Or invite @mediabot:t2bot.io to a room and upload an image. It will tell you the mxc uri.
Most helpful comment
The easiest way to get one is to upload the image to a room somewhere, click on the "..." button that appears when you hover-over the image, click on "View Source", and look for the "mxc://..." URL. Once you have that, then you can use it in the img tag as "\