Please allow, when on the background layer, the orange handle to go beyond the maximum map size such that a tiled background image can fill it all up. Otherwise, the bottom/right edges will not be filled in. (IE, tile over the edge instead of under -- showing only part of an image.)
Low priority, since it requires partial images the way you're doing it. Medium priority if you switch (as I suggest elsewhere) to allowing a specified background image per page, with "center" "stretch" or "tile" as options.