Yes, you want to set the Shape.fill, which is now “green”, to be a Brush that you create of type Brush.Pattern whose Brush.pattern is an HTMLImageElement whose source is your image of stones.
You’ll need to create a separate Brush like this for each of the Parts that are in your Palette. Put them all in a JavaScript object that you use as a map from string to Brush. Define a conversion function (I’ll call it convertNameToBrush) that takes a string and returns the appropriate Brush from this mapping. Add a new go.Binding(“fill”, “pat”, convertNameToBrush) to the Shape. Then you can set data.pat to “stones” or “wood” or whatever you have defined in your string-name-to-pattern-Brush mappings used by the conversion function.
I guess you don’t need the Picture at all, so you can delete it from your template.