$(document).ready(function(){

    function adapter(img) {
		$('#intro').css('backgroundImage', "url('"+img+"')")
    }

    var screenX = screen.width;
            
    if( screenX <= 1024 ) {
	    adapter('1024x768.jpg');
    }
    else if( screenX <= 1366 ) {
        adapter('1366x800.jpg');
    }
    else if( screenX <= 1680 ) {
        adapter('1680x1050.jpg');
    }
    else { // 1920
        adapter('1920x1200.jpg');
    }
});