I recommend you to use an unordered list for your image gallery.
You should use my code unless you want the image to gain instantly 50% opacity after you hover out. You will have a smoother transition.
#photos li {
opacity: .5;
transition: opacity .5s ease-out;
-moz-transition: opacity .5s ease-out;
-webkit-transition: opacity .5s ease-out;
-o-transition: opacity .5s ease-out;
}
#photos li:hover {
opacity: 1;
}