Well, since the map is 20x20, and each tile is 32px, 32x20=640px, so, if you make a map 640x640, it will fit. But, I'd rather do something a little smaller, just to be sure that it will fit...
Try making the map size so that it's odd. This will allow you to have a center tile to place the image on instead of placing the image on an offcenter tile.
Also be sure that world.view is set so that the full map will be displayed. for ecample if you change the map size to 19 by 19 as suggested use world.view="19x19"