img:hover {
  -webkit-filter: grayscale(0%);
  -webkit-transition: .5s ease-in-out;
  -moz-filter: grayscale(0%);
  -moz-transition: .5s ease-in-out;
  -o-filter: grayscale(0%);
  -o-transition: .5s ease-in-out;
  filter: grayscale(0%);
} 

img {
  -webkit-filter: grayscale(100%);
  -webkit-transition: .5s ease-in-out;
  -moz-filter: grayscale(100%); 
  -moz-transition: .5s ease-in-out;
  -o-filter: grayscale(100%); 
  -o-transition: .5s ease-in-out;
  filter: grayscale(100%);
}