Methods for creating some special shadows essentially involve stacking multiple shapes together, then adjusting their stacking order with z-index to finally achieve the desired effect.

HTML:

<li><img src="img/1.gif" alt="img" /></li>

CSS:

li::after {
  content: "";
  width: 90%;
  height: 80%;
  left: 1.6%;
  bottom: 4.6%;
  position: absolute;
  background-color: transparent;
  box-shadow: 0px 8px 20px rgba(0, 0, 0, 0.6);
  -webkit-transform: rotate(-4deg) skew(-8deg);
  -moz-transform: rotate(-4deg) skew(-8deg);
  -ms-transform: rotate(-4deg) skew(-8deg);
  -o-transform: rotate(-4deg) skew(-8deg);
  transform: rotate(-4deg) skew(-8deg);
  z-index: -1;
}
li::before {
  content: "";
  width: 90%;
  height: 80%;
  right: 1.6%;
  bottom: 4.6%;
  position: absolute;
  background-color: transparent;
  box-shadow: 0px 8px 20px rgba(0, 0, 0, 0.6);
  -webkit-transform: rotate(4deg) skew(8deg);
  -moz-transform: rotate(4deg) skew(8deg);
  -ms-transform: rotate(4deg) skew(8deg);
  -o-transform: rotate(4deg) skew(8deg);
  transform: rotate(4deg) skew(8deg);
  z-index: -1;
}

It’s essentially about taking two parallelograms, rotating them left and right respectively, placing them beneath the original image (as shown in the diagram below), and applying a shadow to them, which will ultimately create the shadow effect seen in the image. Click here to view the DEMO