You’ll need a plugin (or set of plugins).
Off the top of my head you could use the jekyll-picture-tag plugin to generate downscaled versions. It’s primary purpose is creating smaller sizes for use with responsive image markup, but it should do what you want.
It expects you to use a {% picture %} tag so it knows what images to use, but you could use another plugin that rewrites images written in Markdown to the format jekyll-picture-tag needs.