The visual templates of the RadDiagram shapes are methods and they are not serialized. This is why the image is not stored in the exported JSON.
Alternatively you can try setting a custom value of the Type
property of each shape, depending on the image inside. When the diagram is loaded from JSON, you could apply a visual template with different image after checking the type of the shape, similarly to the approach that is used in the overview demo: http://demos.telerik.com/aspnet-ajax/diagram/examples/overview/defaultcs.aspx
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