As the guide suggest to add an image to a readme.md markdown file use
![Image of Yaktocat](https://octodex.github.com/images/yaktocat.png)
(Source: https://guides.github.com/features/mastering-markdown/)
But if you want to resize you could use the html img tag instead. Replace the syntax with this
<img src="https://octodex.github.com/images/yaktocat.png" width="100" height="100">