Hi Paul, you can solve this way, I see that you are using boostrap, you do not need the flex property in your css.
<div class="row"> <div class="col-sm-5"> <a href="#"><img src="https://www.jssor.com/demos/img/paint/01.jpg"></a> </div> <div class="col-sm-4"> <a href="#"><img src="https://www.jssor.com/demos/img/paint/02.jpg"></a> <a href="#"><img src="http://lorempixel.com/409/490/technics" class="img-responsive"></a> </div> <div class="col-sm-3"> <a href="#"><img src="https://www.jssor.com/demos/img/paint/06.jpg" class="img-responsive"></a>
and your css
.gallery { min-height: 980px !important;}
Remember to add the download to your html
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
I will explain why you do not need flexibility, Boostrap makes your thml responsive with the .row class, and you can fill it with 12 columns, so you have col-sm- (Number) in your case 5 + 4 + 3 = 12 if you want you can read more in this link grid systemn
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