Remove movement when hovering over images

Removes shakey movement when animating opacity on images with responsive widths.

img {
    opacity: 1;
    -webkit-transform: rotate(0);
    -moz-transform: rotate(0);
    transform: rotate(0);
    -webkit-backface-visibility: hidden;
    position: relative;
}

img:hover {
    opacity: 0.5;
}