Ok. I managed to figure how the script works (it's all new to me).
I changed the string
local bgart = fe.add_artwork( "flyer", flw*0.4321, flw*0.023, flw*0.256, 0);
with
local bgart = fe.add_artwork( "flyer", flw*0.4321, flh*0.0416, flw*0.256, flh*0.562);
And now the flyer fit the showcase. Still not centered, but it's a good start.
Also i've changed the background a bit, adding a cork background, like a bulletin board.