Image link with mouseover

I have an Article with an Element type 'External Site (in a Lightbox)'
I have created this as an image link so I now successfully have a thumbnail image which when clicked opens a URL in the lightbox (using {{link_url::##}})

This all works fine but I need to give my thumbnail a mouseover/hover image.

I can't see anywhere in the options for doing this and I don't know how to do it with CSS (if that's even possible)

Can anyone help? These are the thumbnails:
http://www.la-design.co.uk/index.php/news.html

Many thanks
Corrie

....ok, so i've now coded the CSS to change the background image on hover but:

a) if I leave the 'image link' in the element the whole image is clickable and links to my lightbox but the hover doesn't show because it is the background image and therefore is hidden behind my image link

b) if I remove the image link so only the background images appear, the hover works but the clickable area is reduced to the URL text which just shows at the top of the image :cry:

I need the hover to work and the whole area of the thumbnail image to be clickable...this is driving me nuts!

C

I've made it work!!
Probably a total hash of things that might break in other browsers but here's how:

I created the hover using CSS to change the background image
The I uploaded a totally transparent .png to use as my image link

This way the transparent image gives me the link across the whole image area, but because it's transparent, my hover images are visible :idea:

Please let me know if there's a better way to do this.

Thanks
C