a la.

#img1 {
position: absolute;
background: url('http://i1216.photobucket.com/albums/dd373/XOhxHioHiMixo/meow_zps04436a09.png');
-webkit-transition: all 0.7s ease;
-moz-transition: all 0.7s ease;
-o-transition: all 0.7s ease;
}

#img1:hover {
background: url('http://i.imgur.com/Q5KjFmY.png');
-webkit-transition: all 0.7s ease;
-moz-transition: all 0.7s ease;
-o-transition: all 0.7s ease;
}