That feature is not currently implemented for our jQuery date picker which is used by default, it was used in the jsCalendar Date picker we used previously.
I've put a ticket in my project tracker to remind me to implement it when I get a chance.
But, be advised you cannot use .bmp files on the web it will not work except maybe in IE on windows so no-one uses .bmp files on the web
also note that the correct way to set the path is not like ./somepattoanimage.png but ~/somepathtoanimage.png, ~/ means the root of the site, ./ means go up one folder from here, it is not reliable.
Best,
Joe