$interpolate service does evaluates the {{}} content and take
those value from scope while evaluating scope.
As you want to see the url econded, you are not doing encoding of parameters anywhere. You need to encode your a as well as b in {{}} interpolation using encodeURIComponent of javascript. For that you need to create a wrapper method in scope that will call encodeURIComponent method and return the encode URL, method would be like below.
$scope.encodeContent = function(data){
return encodeURIComponent(data);
}
Thereafter your URL would look like http://www.example.com/images.jpg?a={{encodeContent(a)}}&b={{encodeContent(b)}}
And while attaching it to src of img tag you need to evaluate interpolation first & then you can make that url as trusted as you are doing now.
Markup
<img src="{{trustedUrl(x)}}" width="50" height="50"/>
Code
$scope.trustedUrl = function(url){
var interpolatedUrl = $interpolate(url)($scope)
return $sce.trustAsResourceUrl(interpolatedUrl)
};
Working Plunkr