$(document).ready(function(){

    var max_width = 1086;
    var min_width = 912;
    var bg = $('#main_bg');

    resizeBg();
    $(window).resize(resizeBg);
    
    function resizeBg(){
            
        var left = bg.offset().left;
        var width = $(window).width() - left;
       
        if( width > max_width ) {
    
            bg.width(max_width);
            
        } else if( width < min_width ) {
            
            bg.width(min_width);
            
        } else {
            
            bg.width(width);
            
        }

    }
        
});