The easiest way to do it would be to write
self:add(Tile:new{ image = 'mybackground.png' })
Right at the beginning of your app's onRun method. If you leave out an x or y position, Zoetrope assumes you mean 0. And when you leave off a width and height, a Tile automatically sets itself to the dimensions of the source image. Since you do it before you add anything else to the display list, everything else will appear on top of it -- Zoetrope works on the basis of last-added, first-drawn.
chris