<img src="http://tympanus.net/codrops-playground/assets/images/cssref/properties/mask-type/field.jpg" alt="">

<svg>
  <defs>
    <mask id="svgMask" maskUnits="userSpaceOnUse">
      <polygon fill="#29ABE2" stroke="#000000" stroke-width="4" stroke-miterlimit="10" points=".5,0 1,.5 .5,1 0,.5 "/>
    </mask>
  </defs>
</svg>
body {
  background-color: #111;
  color: #555;
  font-size: 1.1em;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
}

img {
  margin: 20px auto;
  display: block;
  width: 100%;
  max-width: 640px;
  height: 400px;
  -webkit-mask: url(#svgMask) luminance center center / 100% 100% no-repeat;
  mask: url(#svgMask) luminance center center / 100% 100% no-repeat;
}