Let's add a semi-transparent effect to the thumbnails, and each thumbnail will get back to its full opacity when hovered over. This can be easily done via the two built-in CSS class: thumb and thumb-on:
#thumbs .thumb img{ opacity:0.6;filter:alpha(opacity:60); }
#thumbs .thumb-on img{ opacity:1;filter:alpha(opacity:100);}
If there are lots of thumbnails and the containing block does not have enough room to show them, you can consider showing the thumbnails in multiple columns or rows. Another solution is to make the slider work together with VeryUtils Thumbnail Slider.