Well, I think I found a way to do that. Don’t no if it’s the best one. But in the end I didn’t like it at the images that were not square. So I made just one modification to have a mouseover effet on them.
Here is how I managed to have the shadow the same the afg gallery:
img.widget-img {
-webkit-box-shadow: rgba(0,0,0,0.6) 0 0 7px;
-moz-box-shadow: rgba(0,0,0,0.6) 0 0 7px;
box-shadow: rgba(0,0,0,0.6) 1px 1px 7px;
margin-top: 5%;
opacity: 1;
}