I’m not sure what might be happening.
If I open up the org chart editor sample (chosen because it has images): http://gojs.net/latest/samples/orgChartEditor.html
And open the developer console, and paste in this:
document.body.appendChild(myDiagram.makeSvg({
scale: .5,
showGrid: true,
showTemporary:true,
position: new go.Point(0,0),
size: new go.Size(8000, 8000)
}));
(same as your code but I changed the scale to .5 to see it better)
And scroll down on the page, I appropriately see the generated SVG with images included. This seems to work on Chrome, Firefox, and IE11.
What's different about this case?