a .button_2 
{
    background-image: images_diagnostic/2_s1.png;     
}

a:hover .button_2
{
    background-image: images_diagnostic/2_s2.png; 
}

a:active .button_2 
{
    background-image: images_diagnostic/2_s3.png;
    width: 52px;
    height: 39px;
    border: 0px;   
}