On every version of Joplin I've tried (Mac, Linux, iOS), a markdown command like the following displays correctly:
<img src="https://reallybigimage.jpeg" width="200" />
When synched across OSes and devices, all works as I expect: it displays the image in the note.
Question: where is that (cached?) image stored? It doesn't seem to be in the /resources directory. The image is displayed too quickly (and it works offline) so I know it's not pulling it from the original every time.
I like how it doesn't appear to burden my disk space, but I'm worried I could lose content someday if that original URL goes offline.