button over image:

<div class="img-wrapper">
    <img class="img-responsive" src="your_image_goes_here">
    <div class="img-overlay">
      <button class="btn btn-md btn-success">Button</button>
    </div>
</div>

.img-wrapper {
  position: relative;
 }

.img-overlay {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  text-align: center;
}