Emebed image in a task

I would like to embed an image in a task form by using the directive img and the attibute ng-src.
In one variable of the instance (named documentUri) is stored the URI of the image (eg: “http://domain/image”.

I cannot succeed with any of the following:

<img src="{{camForm.variableManager.variable('documentUri').contentUrl}}" width="100%" class="img-responsive">

<img ng-src="{{documentUri}}" width="100%" >

<img src="{{documentUri}}" width="100%" >

The binding works perfectly for the input directives.

Any support is welcome.