img {
  -webkit-filter: grayscale(0%); /* For Webkit browsers */
  -webkit-transition: .5s ease-in-out; /* For Webkit browsers */
  -moz-filter: grayscale(0%); /* For Firefox */
  -moz-transition: .5s ease-in-out; /* For FireFox */
  -o-filter: grayscale(0%);
  -o-transition: .5s ease-in-out;
} 
 
img:hover {
  -webkit-filter: grayscale(100%); /* For Webkit browsers */
  -webkit-transition: .5s ease-in-out; /* For Webkit browsers */
  -moz-filter: grayscale(100%); /* For Firefox */
  -moz-transition: .5s ease-in-out; /* For Firefox */
  -o-filter: grayscale(100%); 
  -o-transition: .5s ease-in-out;
}