You can try this. Just paste it into the custom css section in the admin panel, and add you image urls.

body.ow.base_sign_in {
    background-image: url("your image url");
    background-size: 100% 100%;
    background-attachment: fixed;
    background-repeat: no-repeat;
}

.ow_sign_in_wrap h2 {
    font-size: 0px;
    background-image: url("your image url");
    height: 100px;
    background-size: 100% 100%;
    background-repeat: no-repeat;
}

Try adding the to the custom css in the admin panel as well. You can change it to the setting that works for you. The original setting is 200px

.ow_sign_in_wrap {
    top: 100px;
}

I have added code that you give to the "Custom CSS" in admin panel but nothing has changed










but I have managed to shift slightly upward sign box by changing the size of the "top" with 120px in /ow_system_plugins/base/views/controllers/user_standard_sign_in.html.
And I think the code in the "user_standard_sign_in.html" just the same as you give.











Thanks for your help