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.maintxt {position: relative;}
.maintxt > img, .overlay-text {position: absolute;}
<div class="container">
<div class="row">
<div class="col-xs-12 col-sm-6 col-md-8 col-centered">
<div class="maintxt">
<img src="background.png" class="img-responsive">
<span class="overlay-text">My super text.</span>
</div>
</div>
</div>
</div>
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.maintxt {
background-image: url(images/background.png);
background-size: cover;
}
<div class="container">
<div class="row">
<div class="col-xs-12 col-sm-6 col-md-8 col-centered">
<div class="maintxt">
My great text.
</div>
</div>
</div>
</div>