JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap">
    <img src="http://alpatriott.ru/works/primer/images/romashki.jpg" alt="" width="300" height="300">
</div>

CSS

body{
padding:25px;
background:#222;    
}

.wrap{
display: inline-block;
margin: 25px;
position:relative;
overflow:hidden;
border-radius: 8px;
}

.wrap>img{
display:block;
}

.wrap:before{
content:'a';
position:absolute;
top:-100%;
left:0;
width:100%;
height:100%;
background: rgba(255,255,255,0.3);
border-top: 100% solid rgba(255,255,255,0.3);
border-left: 50% solid rgba(255,255,255,0.3);    
}