#link11 {
     background-image: url(../bilder/collagen.png);
}

#link11:hover {
     background-image: url(../bilder/collagen-hover.png);
}

#link12 {
     background-image: url(../bilder/fotos.png);
}

#link12:hover {
     background-image: url(../bilder/fotos-hover.png);
}
#link21 {
     background-image: url(../bilder/skizzen.png);
}

#link21:hover {
     background-image: url(../bilder/skizzen-hover.png);
}