Inside News, over button news move!

Hi everyone, i had a problem with my old version of cutenews and updating to the last version the problem still. maybe is not a problem in the cutenews system.

here what happen :

I have a download button inside my news, when i move the mouse cursor the button image change to other,

when it change the news below move few pixels down and when i move the cursor out the image the news move again few pixels up.

here in the site where you can check it : http://www.mkmugen.com.ar/download.php

some extra info :

Im testing it in Firefox.

this is the code im using for change the image :

<img onmouseout="this.src='img_index/download_1.gif'" onmouseover="this.src='img_index/download_2.gif'" src="img_index/download_1.gif">

thanks