Resize image in the wiki of GitHub using Markdown

Updated:

Markdown syntax for images (external/internal):

![test](https://github.com/favicon.ico)

HTML code for sizing images (internal/external):

<img src="https://github.com/favicon.ico" width="48">

Example:

Old Answer:

This should work:

[[ http://url.to/image.png | height = 100px ]]

Source: https://guides.github.com/features/mastering-markdown/