Question

I figure that using a div with CSS rounded corners to clip an image will make a page load faster than using an image clipped in Photoshop, then exported with transparency as PNG-32.

Answer 1

Make sure you make it cross-browser compatible:

img {
    -webkit-border-radius:10px;
    -moz-border-radius:10px;
    border-radius:10px;
}

The shorthand for the border-radius property is:

border-radius: top-left top-right bottom-right bottom-left;

So a value of 10px 30px 10px 30px would round the top-right and bottom-left corners a lot more then the other two:

-moz-border-radius-topleft:10px;
-moz-border-radius-topright:30px;
-moz-border-radius-bottomright:10px;
-moz-border-radius-bottomleft:30px;
-webkit-border-radius:10px 30px 10px 30px;
border-radius:10px 30px 10px 30px;