As far as I know, there is currently no “official” way to add a caption to an image.
I would suggest, that an image with nonempty alt text, occurring by itself in a paragraph, will be rendered as a figure with a caption. The image’s alt text will be used as the caption. Inline Markdown in the alt text should be interpreted.
That means that following Markdown snippet
![This is the caption](image.png)
![[image.png|This is the caption]]
will be rendered as something like
<img src="path/to/image.png" />
<figcaption>This is the caption</figcaption>
This is the way pandoc with the implicit_figures extension handles figure captions in Markdown documents.
Concerning implementation details I would suggest to make this behavior optional as an extension.
I already use a very similar workaround (see my workaround link above). The biggest disadvantage of this approach for me, however, is that inline Markdown in the alt text will not be interpreted, so you can not use e.g. inline math in caption text.