import hou
items = hou.selectedItems()
editor = hou.ui.paneTabOfType(hou.paneTabType.NetworkEditor)
images = list(editor.backgroundImages())
for item in items:
    if isinstance(item, hou.NetworkBox):
        nodes = item.nodes()
        nodes = hou.sortedNodes(nodes)
        for i in range(len(nodes)):
            node = nodes[i]
            pos = node.position()
            image = hou.NetworkImage()
            image.setPath('$HIP/images/'+str(i+1)+'.png')
            x1 = - 0.4
            y1 = 0
            x2 = x1 + 0.4
            y2 = y1 + 0.4
            image.setRelativeToPath(node.path())
            image.setRect(hou.BoundingRect(x1,y1,x2,y2))
            images.append(image)
editor.setBackgroundImages(images)