Markdown:

    ![alttext](example.jpg)

RST:

    .. image:: example.jpg
      :alt: alttext

The author prefers RST because:

    ... you can extend Sphinx with new text objects!
    Say you that instead of an <image>, you want
    a <figure> with a <figcaption> ...

What I would do:

        ![alttext](example.jpg)[caption]

And write a renderer which supports this syntax to create a figure with a figcaption. A markdown renderer is simple enough to write your own or extend an existing one.