[ruslan_mart]

CSS:

.bg {
   background: #000;
   display: none;
   height: 100%;
   left: 0%;
   opacity: 0.7;
   position: fixed;
   top: 0%;
   width: 100%;
   z-index: 5;
}

.loading {
   bottom: 0%;
   left: 0%;
   margin: auto;
   position: fixed;
   right: 0%;
   top: 0%;
   z-index: 6;
}

HTML:

<div class="bg"></div>
<img alt="" class="loading" src="loading.gif" />

JS:

function preLoader(a) {
   $('.bg, .loading')[a ? 'fadeIn' : 'fadeOut'](500);
}

/***********************/

preLoader(true);
$.post('test.php', {foo: true}, function(data) {
   preLoader(false);
});