You could just use some HTML in your Markdown:

<img src="drawing.jpg" alt="drawing" width="200"/>

Or via style attribute ( not supported by GitHub )

<img src="drawing.jpg" alt="drawing" style="width:200px;"/>

Or you could use a custom CSS file as described in this answer on Markdown and image alignment

![drawing](drawing.jpg)

CSS in another file:

img[alt=drawing] { width: 200px; }