Defined Edges With CSS3 Filter Blur

You could put it in a <div> with overflow: hidden; and set the <img> to margin: -5px -10px -10px -5px;.

Demo:

Output

CSS

img {
    filter: blur(5px);
        -webkit-filter: blur(5px);
        -moz-filter: blur(5px);
        -o-filter: blur(5px);
        -ms-filter: blur(5px);
    margin: -5px -10px -10px -5px;
}

div {
    overflow: hidden;
}
​

HTML

<div><img src="http://placekitten.com/300" />​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​</div>​​​​​​​​​​​​