non so se funge, prova:

codice:
css

a{ 	
    padding: 0px 19px; 	                 
    text-decoration: none; 	                 
    background-image: url(../images/bottone.gif); 	                 
    background-repeat: no-repeat;  
}    
       
a.uno{ 	                  
    background-image: url(../images/bottone_Hover.gif);  
}    

html
<a onmousedown="this.className='uno'"onmouseup="this.className=''">aaaaaaaaa</a>