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="{{interface == 'UP' && 'green-checkmark.png' || 'big-black-X.png'}}"/>
Note : the quotes (ie 'green-checkmark.png') are important here. It won't work without quotes.
plunker here (open dev tools to see the produced HTML)