cust-src-right.js

angular.module("test", [])
  .controller("testCtrl", function($scope) {
    $scope.value = "";
  })
  .directive('custSrc', function() {
    return {
      restrict: "A",
      scope: {
        value: "=custSrc"
      },
      link: function(scope, element, attrs) {
        scope.$watch("value", function(value) {
            // could also be function(){return scope.value;}
          element.attr('src', value);
        });
      }
    };
  });

cust-src-wrong.js

angular.module("test", [])
  .controller("testCtrl", function($scope) {
    $scope.value = "";
  })
  .directive('custSrc', function() {
    return {
      restrict: "A",
      scope: {
        value: "=custSrc"
      },
      link: function(scope, element, attrs) {
        scope.$watch(scope.value, function(value) {
          element.attr('src', value);
        });
      }
    };
  });