The common syntax for the Markdown image is `` (see [md-images]).
Not sure if the alt-text should be considered optional by the markdown parser (in HTML it is optional) . Alt-text is useful for textual labeling of the image and used by screen readers too. IIRC, specifying the alt-text is indeed considered a best practice.
[md-images]: https://daringfireball.net/projects/markdown/syntax?spm=a2c65.11461447.0.0.6...