Edit in JSFiddle

<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/36/Hopetoun_falls.jpg/450px-Hopetoun_falls.jpg">

img{
    -webkit-transition: 1s;
    -webkit-clip-path: circle(60px at center);     
}

img:hover{    
    -webkit-clip-path: circle(120px at center);   
}