It’s hacky and not always pretty, but surround the image with a <kbd> tag.

Before

<img src="https://i.stack.imgur.com/CtiyS.png">

After

<kbd>
  <img src="https://i.stack.imgur.com/CtiyS.png">
</kbd>

And it renders like this:

Surrounding the markdown image syntax might not work for some markdown implementations. If the following does not work

<kbd>![alt-text](https://example.com/image.png)</kbd>

Try embedding the image as an HTML <img> tag instead

<kbd><img src="https://example.com/image.png" /></kbd>