The site looks really nice and lots of changes already in place. I would suggest you wrap this in three_col instead of four_col since you have four images ( I have six images and hence I used two_col). And to avoid the image resizing automatically, you will need to use the following custom css –
#our-clients .client-list img {
width: auto;
}
#our-clients .client-list .threecol:hover img.hover { opacity: 1; }