In the case of a 'missing' image there are several things to consider:
When XAML can't locate a resource it might ignore it (when it won't throw a XamlParseException)
The resource must be properly added and defined:
Make sure it exists in your project where expected.
Make sure it is built with your project as a resource.
(Right click → Properties → BuildAction='Resource')

Another thing to try in similar cases, which is also useful for reusing of the image (or any other resource):
Define your image as a resource in your XAML:
And later use it in your desired control(s):