how to add grid on top of image which is used as panel in google app script?

how to add grid on top of image which is used as panel in google app script?

I am creating a panel having grid with controls like text box,label etc.
which should appear on image.
For that I have main panel
var mainPanel =
app.createVerticalPanel().setId("mainPanel").setVisible(false);
on this main panel I have added an image
var file =getFileFromFolder(existedFileName);
var fileID = file.getId();
var url = 'http://....;
mainPanel.add(mainImage);
Now on this image I need to add some controls so I have done something
like this...
var requiredGrid = app.createGrid(1,
2).setVisible(false).setId("requiredGrid");
mainPanel.add(requiredGrid);
but this adds requiredGrid below the image panel. Now I want to add this
on image i.e. image should come as a background for the requiredGrid.