You need 8 parts for showpoly (9 including img id).
PHP Code:
showpoly(4000,{0,0,screenwidth,0,screenwidth,screenheight,0,screenheight});
Takes up the whole screen. You need to give an x and y for each seperate corner (top left, top right, bottom left, bottom right).
I think it's something like, if the screen is like this
x1y1...............................x2y2
...........................................
x4y4...............................x3y3
showpoly(ID,{x1,y1,x2,y2,x3,y3,x4,y4});