Instead of src you need ng-src.
AngularJS views support binary operators
condition && true || false
So your img tag would look like this
<img ng-src="https://stackoverflow.com/questions/13999659/{{interface =="UP' && "https://stackoverflow.com/questions/13999659/green-checkmark.png" || 'big-black-X.png'}}"/>
Note : the quotes (ie “https://stackoverflow.com/questions/13999659/green-checkmark.png”) are important here. It won’t work without quotes.
plunker here (open dev tools to see the produced HTML)