You want a way to center any object to the middle of the page (horizontal centering)?

index.html

<div id="first">
  <div id="second">
    Item you want centered
  </div>
</div>

for example:

<a href="mailto:info@email.com?Subject=Web%20Page" target="_top" id="first">
  <img src="photo.png" alt="photo">
</a>

style.css

#first {
  position: absolute;
  left: 50%;
}

#second {
  position: relative;
  left: -50%;
}

or, from the example:

#first {
  position: absolute;
  left: 50%;
}

#first img {
  position: relative;
  left: -50%;
}