Basically, just add the following properties to the CSS:
.mask-loading {
      position: absolute;
      top: 0px;
      left: 0px;
      z-index: 1000;
      background-color: #000;
      opacity: 0.5;
      width: 100%;
      height: 100%;
}
This way you are adding color, opacity, size and position to your div.
Below is an example of how it works. It is not your model, but it is possible to understand what you want.
$('#loader').click(function() {
  $('.mask-loading').show();
  setTimeout(function() {
    $('.mask-loading').hide();
  }, 5000);
});
.mask-loading {
  position: absolute;
  top: 0px;
  left: 0px;
  z-index: 100;
  background-color: #000;
  opacity: 0.5;
  width: 100%;
  height: 100%;
  display: none;
}
img {
  top: 500px;
  z-index: 1000;
  background-color: #fff;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="mask-loading">
  <figure>
    <img src="http://www.mtlexs.com/images/reload.gif" alt="carregando...">
    <figcaption class="loading-text"></figcaption>
  </figure>
</div>
<div>
  sua página aqui
</div>
<button id="loader">Loader</button>
 
 
There are several ways to do what you want. However, as I don’t know your code, I added a way that I think should work for you.
							
							
						 
I don’t get it. The way you want it is the image or the way it is today is the image?
– Randrade
Yes, the shape I want is the image
– João Vitor
I did not understand very well, lacked your jquery code and css
– Lennon S. Bueno