LightPolice

I found the solution 🙂
I have modified the line 323 in flickr-justified-gallery.php

$ris .= '<img class="grayscale" alt="' . htmlspecialchars($photo['title'], ENT_QUOTES, 'UTF-8')

I can now modify the appearance of the gallery, declaring class “grayscale” in the css file 🙂
Declaration in my style.css

.grayscale {
 -webkit-filter: grayscale(70%);
 -moz-filter: grayscale(70%);
 -ms-filter: grayscale(70%);
 -o-filter: grayscale(70%);
 filter: grayscale(70%);
 filter: gray; /* IE 6-9 */
/*transition:all 0.4s ease-in-out;
-webkit-transition:all 0.4s ease-in-out;
-moz-transition:all 0.4s ease-in-out;
-ms-transition:all 0.4s ease-in-out;
-o-transition:all 0.4s ease-in-out;*/
}
.grayscale:hover {
  -webkit-filter: none;
  -moz-filter: none;
  -ms-filter: none;
  -o-filter: none;
  filter: none;
}

The effect can be seen on my website, the effect can be seen on my website, does not work on MS-IE 😛

http://www.tomlight.pl