You mean images in advert1-advert4? Go to module manager you will find custom HTML module in each of them, you enter modules config to change their content.
If you truly want to change their background, you can add css like following at bottom of template_css.css :
.mp_advert1{
background :...
}
.mp_advert2{
background :...
}
.mp_advert3{
background :...
}
.mp_advert4{
background :...
}