How have you defined your node template?
I suspect that the images have some margin around where the contents of the image are drawn. As far as GoJS is concerned, each Picture is a rectangular area where the image is drawn, and there is no way to tell what is drawn within that rectangle. You can tell if that is the case by looking at the image using an image editor to see if the edges of the truck and the satellite dish touch the sides of the image.
If it is the case that the image appears to have a margin, not the Picture, then there might be some imperfect solutions. One thing you could try is setting on your Link template(s) the following properties:
layerName: "Background", fromShortLength: -20, toShortLength: -20,
This presumes that you are not already using the “Background” Layer for other purposes. You may want to experiment with different values than -20.