Rollover on images

Best way would be to use some custom css

The css code below will do a opacity hover over effect.

#wpsimplegallery li img {
    opacity: 0.5;
    -webkit-transition: opacity 1s ease;
    -moz-transition: opacity 1s ease;
    -ms-transition: opacity 1s ease;
    -o-transition: opacity 1s ease;
    transition: opacity 1s ease;
}
#wpsimplegallery li img:hover {
    opacity: 1;
}