How to bind the current value of an attribute image in a ViewModel using the tag img (Knockout.js)?

How to bind the current value of an attribute image in a ViewModel using the tag img (Knockout.js)? I want to see the current value (eg, 'sort_down.png'), but the attribute src of the image tag doesn't change ('no_sort.png'). Help me to solve the problem in this code. I also used the Knockout method ko.computed but the code doesn't work.

<img data-bind="attr:{src: image}"></p>
<div data-bind="click: sortTableByName"></div>

function ViewModel() {

                var self = this;

                self.articles = ko.observableArray(model);

                self.asc_sort_string = ko.observable(false);
                self.desc_sort_string = ko.observable(false);

                self.image = ko.observable('no_sort.png');

                self.update = function(){
                    // ... code
                };

                self.sortTableByName = function(){
                    if (self.desc_sort_string == false) {
                       self.asc_sort_string = false;
                       self.desc_sort_string = true;
                       self.image = 'sort_down.png';
                    }else{
                       self.asc_sort_string = true;
                       self.desc_sort_string = false;
                       self.image = 'sort_up.png';
                    }
                };


            };

            var viewModel = new ViewModel();

            ko.applyBindings(viewModel);

            viewModel.update();