html coding help for tumblr?

so im using this code in my tumblr html

img{
-webkit-filter: grayscale(100%);
}
img:hover{
-webkit-filter: saturate(100%);

z-index: -9999999999999999999999999px;

-webkit-transition: all 0.9s ease-in-out;

-moz-transition: all 0.9s ease-in-out;

-o-transition: all 0.9s ease-in-out;

-ms-transition: all 0.9s ease-in-out;

transition: all 0.9s ease-in-out;}

it turns my photos black and white, and when i hover over them it changes to color. I do not want to change this HOWEVER how may i edit it so that the photos fade into color rather than doing it immediately