You could just use some HTML in your Markdown:
Or via style attribute (not supported by GitHub)
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; }
With certain Markdown implementations (including Mou and Marked 2 (only macOS)) you can append =WIDTHxHEIGHT after the URL of the graphic file to resize the image. Do not forget the space before the =.
![](./pic/pic1_50.png =100×20)
You can skip the HEIGHT
![](./pic/pic1s.png =250x)