iMovie Cover Picture and Icon image

Also the image chosen for the file icon I would like to change. There is a posted answer to use Get Info and paste a new image into the Preview window but I have had no success trying to do this.
Has anyone come up with a solution to what ought to be a menu option?
I don't mind delving into the guts of the machine and substituting files if I knew where to look but I've been unable to find the relevant folder.