How to change image source with a default one (empty)?

Hello,

I need help about the code below.

When I go to the next step I want to change the custom image from one to a default one (empty). 

$scope.step6 = function(){
$scope.app.params.descrizioneAnimazione1 = "app/resources/Uploaded/Descrizioni/Immagine4-2.png";
$scope.app.params.descrizioneAnimazione2 = "app/resources/Uploaded/Descrizioni/Immagine5-2.png";


$scope.step7 = function(){
$scope.app.params.descrizioneAnimazione1 = '';
$scope.app.params.descrizioneAnimazione2 = '';

At the moment when I change the step the images remains the same as the previous step and I would like that if the step has no image as a source, I don't want to see any images.

 Thanks in advanced!

Filippo