index.html

<img src="http://farm4.static.flickr.com/3647/3320066629_c26343104f.jpg" alt="yeahh">

style.scss

@import "compass";
img{
  @include transition-property(all);
  @include transition-duration(.5s);
  @include transition-timing-function(ease-in);

  -webkit-filter: grayscale(100%);
  -moz-filter: grayscale(100%);
  -ms-filter: grayscale(100%);
  -o-filter: grayscale(100%);
  filter: grayscale(100%);
  filter: Gray();
  &:hover{
    -webkit-filter: grayscale(0%);
    -moz-filter: grayscale(0%);
    -ms-filter: grayscale(0%);
    -o-filter: grayscale(0%);
    filter: grayscale(0%);
    filter: none;
  }
}