I've tested this on Outlook 2013 Desktop, and the ALT text does not display when the height and width are not expressed as pixels.

So this will not show ALT text in Outlook 2013:

<img src="image.gif" alt="This is alt text" style="font-family: Helvetica, Arial, Verdana, Trebuchet MS, sans-serif; font-size: 16px; height: auto; width: 100%;"/>

But this will show the ALT text in Outlook 2013:

<img src="image.gif" alt="This is alt text" style="font-family: Helvetica, Arial, Verdana, Trebuchet MS, sans-serif; font-size: 16px; height:310; width:600;"/>

Anyone know of any possible work arounds?