#by defining the rows and columns you are defining which of the suplots you are adressing
You can add an image as the background to a graph with:
#using input image sizes as the
#axes lengths for the graph
#defining height and width of the graph
You would have to adapt this last piece to add the layout directly to the subplot and not for the whole graph.
I haven’t done this before and the syntax is a bit different in that case, so you would have to fiddle around with that a bit.