embedding images
To embed an image you need a URL to it. If you git-clone your wiki locally you can then just add the image to the wiki directory and then link to it using a relative link. As shown below, using standard markdown.
But, if you want to change the width or height of the image you have to use HTML, with the width and height attributes. Yes, you can embed bits of HTML in your markdown. It will work:
<img src="img/bros.jpg" alt="Beagle Bros" width="100"/>
If you want to do everything from github itself, one way is to upload your images to a dummy issue (like this one) and then use that url.
