[Andrei]

exec, a если так ?

$.ajax({
type: "POST",
url: "/rand_header.php",
data: "userHour="+myHour,
success: function(pic){
    myImage = new Image();
    function write(){
        $('#header').css('backgroundImage', 'url('+myImage.src+')');
        $('#header').animate({'opacity':'1'}, 230); 
    }
 
    myImage.onload = write;
    myImage.src = '/themes/images'+pic;
}
});