@docroach - Unfortunately this is not currently possible using out-of-the-box functionality in Miro.
A few options are:
iFrame
You could embed an image using basic iFrame HTML code, however, the image won’t display by default (without even more work of setting up an actual page to host the file along with Open Graph meta tags - Google it). In order for the user to see the image, they would have to click the “play” icon. To try it, follow the insetructions in the Embedding Media to Boards Help Centre article using this iFrame code:
<iframe src="https://i.picsum.photos/id/237/200/300.jpg?hmac=TmmQSbShHz9CdQm0NkEjx1Dyh_Y984R9LpNrpvH2D_U"></iframe>
To have the image change, you would then replace the “300.jpg” file with another image with the same name.
Miro Developer Platform
It may be possible to create a custom a web-plugin to accomplish your desired outcome using the Miro Developer Platform. You would would probably want someone with a development background to review the developer platform documentation.
Wish List Idea
Add this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know. If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.