.owl-carousel img{
  -webkit-filter: grayscale(100%);
  -moz-filter: grayscale(100%);
  -ms-filter: grayscale(100%);
  -o-filter: grayscale(100%);
  filter: grayscale(100%);
  filter: gray; /* IE 6-9 */
}
.owl-carousel img:hover {
  -webkit-filter: none;
  -moz-filter: none;
  -ms-filter: none;
  -o-filter: none;
  filter: none;
  filter: none; /* IE 6-9 */
}