Assuming you reference the class library from the WPF application you can reference and display the image in the WPF application with the following XAML:
<Image Source="/ClassLibraryName;Component/images/myimage.png"/>
The important thing here is "ClassLibraryName" which is the assembly name for your class library. "/images/myimage.png" is the path to your image.
You can find out more about WPF pack URIs here.
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