The diagram automatically saves the Image property of nodes in XML by serializing the images in base64 format. If you are using controls hosted in DiagramNodeAdapters and these controls display images, either use relative file paths as images sources and copy the image files too with the xml, or load all images inside hidden ShapeNodes and assign them back to the hosted controls when a diagram is loaded.

I meant you could copy the external image files together with the diagram xml if using relative URLs.

Another option is to change your tool's file format to a zip archive containing the xml and image files, as shown here -
https://msdn.microsoft.com/en-us/library/system.io.packaging.zippackage.aspx

If you prefer storing the image data as a Tag value of nodes, you can use the FromImage method of MindFusion.Diagramming.Converter to get base64 encoded image bytes, and after loading the diagram call ToImage to load images from the node Tag values.

I hope that helps,
Stoyan