Is there an equivalent of
#+attr_html: :width 500px for Markdown format ?
The markdown exporter doesn't appear to export image sizes defined by
ox-gfm exporter doesn't either)
so, you could either include raw html in the org file, which does get exported...
#+html: <img src="img/test.png" width="600px">
or if you have pandoc installed, you can use it as an external process to convert from org to markdown. The conversion will include image properties defined in the
pandoc -i test.org -o test.md