Give this a shot. Upload your image to your theme graphics, and copy the url, and paste it into the image url css below. Put all of this into the Simplicity theme's custom css.
.ow.base_sign_in .ow_sign_in_cont {
background: transparent none repeat scroll 0% 0%;
}
body.ow.base_sign_in {
background: transparent none repeat scroll 0% 0%;
}
body.ow.base_sign_in div.ow_page_wrap{
background-image: url("your image");
background-size: 100% 100%;
background-attachment:fixed;
}