When you create a BitmapImage in a ResourceDictionary, you have to use a string to set the path of the resource:
This can easily break if you move around the assets in your project. A better way would be to declare the BitmapImage in XAML, but set the resource through the editor like this:
However, it seems you can't see resources in this window; it's read-only. It would save a lot of time, and avoid potential issues in the future, if we could just set the resources in this window.

That view is read-only and displays the dependencies for your XAML.

One approach you could take is to expose a dictionary of images in your data context and bind to it from your XAML. The dictionary could be fully populated within Unity.

Would that solution work for your needs?