The onBeforeCellRender event is fired during image export. It lets you set custom cell background color.
DayPilot.Calendar.onBeforeCellExport(args);
The args object is the same instance that is used in onBeforeCellRender (which is called right before onBeforeCellExport).
dp.onBeforeEventExport = function(args) {
args.backColor = "white";
};