I used Zack Argyle's suggestion above to get this, which I find very elegant:
CSS:
.active {
background-position: 0 -46px !important;
}
HTML:
<button ng-click="satisfaction = 'VeryHappy'" ng-class="{active:satisfaction == 'VeryHappy'}">
<img src="images/VeryHappy.png" style="height:24px;" />
</button>
<button ng-click="satisfaction = 'Happy'" ng-class="{active:satisfaction == 'Happy'}">
<img src="images/Happy.png" style="height:24px;" />
</button>
<button ng-click="satisfaction = 'Indifferent'" ng-class="{active:satisfaction == 'Indifferent'}">
<img src="images/Indifferent.png" style="height:24px;" />
</button>
<button ng-click="satisfaction = 'Unhappy'" ng-class="{active:satisfaction == 'Unhappy'}">
<img src="images/Unhappy.png" style="height:24px;" />
</button>
<button ng-click="satisfaction = 'VeryUnhappy'" ng-class="{active:satisfaction == 'VeryUnhappy'}">
<img src="images/VeryUnhappy.png" style="height:24px;" />
</button>