[UPDATE]

A pure CSS solution would be to use CSS filters:

img:hover {

    filter: brightness(1.5);

}

Here we add 50% brightness to all images on hover.

Why not? You can always set the background of the parent container to #fff (white) and then lower the opacity of the image.

HTML:

<div class=white>

    <img src=image.jpg alt=Image />

</div>

CSS:

.white { background: #fff; }

img:hover {

    opacity: 0.5;

    filter: alpha(opacity=50);

}

Another solution is to use a JavaScript library, such as Pixastic.