HTML
====

<li class="selected"><a class="mainpage" ...
<li><a class=contact ...

CSS
===

#menu2 .mainpage {
    background-image: ...
}

#menu2 .contact {
    background-image: ...
}

#menu2 .selected .mainpage {
    background-image: ...
}

#menu2 .selected .contact {
    background-image: ...
}