Do you want users to be able to upload an image directly into the stencil? Drag and drop a stencil element into a diagram and be able to view the original image in the LightBox? See this simple CodePen example.
Upload the image to the stencil, drag the image into the diagram and click on it to display it in a LightBox.
The source code of this demo is available as part of the JointJS+ commercial license. Don't have a license yet? Start a trial and use the source code of this and many other demos for free, with no obligations, for 30 days.