SOURCE

<div class="img-container">
    <div class="img-left"></div>
    <div class="img-right"></div>
</div>

.img-container{
    position: relative;
    width: 200px;
    height: 100px;
    border: 5px solid #40BCFF;
}
.img-left{
    background: url(http://demo.imatlas.com/img/left.jpg);
    background-size: cover;
    width: 100%;
    height: 100px;
}
.img-right{
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100px;
    background: url(http://demo.imatlas.com/img/right.jpg);
    background-size: cover;
    -webkit-mask-image: linear-gradient(115deg, transparent 50%, white 50%);
}