how to resize image in order to make it full screen

// RUN WHEN DOM IS READY
$(function(){
   bg_adjustments = function() {
        var bgwidth = $("#bg").width();
        var bgheight = $("#bg").height();
        if($("#bg").attr("width")){
             bgwidth = $("#bg").attr("width");
        }
		if($("#bg").attr("height")){
		   bgheight = $("#bg").attr("height");
		}
	    var aspectRatio  = bgwidth / bgheight;
		var error_height=0;
		if ($("#errorBody")){
		   error_height = $("#errorBody").height();
		}
        var mb_height = $(window).height() - $("#header").height() - $("#header-additional").height() - $("#footer").height()+error_height;
        var mb_width=$(window).width();
        if(mb_width<1024){
           mb_width=1024;
        }
		var minHeight = 613;
        if(mb_height<minHeight){
           mb_height=minHeight;
        }
		// this is defined for the problem if the content is too long
		if(mb_height< ($("#content_container").height())){
		   mb_height = $("#content_container").height();
		}
        
		// define the background width and height 
        $("#bg").css("height",function() {			
                return mb_height;
	    });
		
		var bg_resize_width= mb_height*aspectRatio;
		
				
		
        $("#bg").css("width",function() {
                return bg_resize_width;
	    });
		
        //end define

		//define the div of background 
        if(bg_resize_width>mb_width){
		  $("#main_container").css("overflow","hidden");   
		}
			    $("#main_container").css("height",function() {
			return mb_height;
	    });
		$("#main_container").css("width",function() {
			return mb_width;
	    });
        //end define
		
		//define the content's position
        $("#content_container").css("top",function() {   
		   var mb_top = $("#header").height() + $("#header-additional").height();
		   return mb_top;
	    });

        $("#content_container").css("left",function() {   
		   var mb_left = ($(window).width() - 1000)/2;//min window width:1024px;
		   if (mb_left<12){
			   mb_left=12;
		   }
		   return mb_left;
	    });
		//end define
		
		//define the Touts's position
		var content_bottom=$("#content-bottom");
		if (content_bottom){
			content_bottom.css('top',function(){
					var minHeight = 613;
					if(mb_height<minHeight){
					   mb_height=minHeight;
					}
					var newTop = 0;
					if( mb_height > 613){
						newTop = ( mb_height - 600)/2;
					}
					return newTop;
			});
		}
		//end define
				
    }
    $(document).ready(bg_adjustments);
    $(window).resize(bg_adjustments);
	setTimeout(bg_adjustments, 500);
    
});