I think the answer is, you can't.

The Musing text box doesn't appear to accept html commands. Something like this—Google—only shows up as text rather than a link.

Images must be online or modified through Steemit (thus providing an online link).

Neither does it appear to work in markdown, either: Pixabay.

I have to say, though, it would be very useful, and if I'm missing something and someone has figured out, I'd be interested to know how, too.