Typically an edit in your theme’s style.css file should help. I use the “opacity” of the image this way:
.photonic-flickr-image img, .photonic-picasa-image img, .photonic-500px-image img, .photonic-smug-image img {
opacity: 0.75;
}
.photonic-flickr-image a:hover img, .photonic-picasa-image a:hover img, .photonic-500px-image a:hover img, .photonic-smug-image a:hover img {
opacity: 1;
}
This keeps it light by default and darkens it upon hover. For the reverse I believe you will have to add some more stuff:
.photonic-flickr-image a, .photonic-picasa-image a, .photonic-500px-image a, .photonic-smug-image a {
background-color: #000 !important;
margin: 5px !important;
}
.photonic-flickr-image a:hover img, .photonic-picasa-image a:hover img, .photonic-500px-image a:hover img, .photonic-smug-image a:hover img {
margin: 0 !important;
}
Make sure you put the “!important” (or use a higher CSS specificity for the selectors above).