This is a java script, and css approach to setting a background image on the forgot password page. See the attached file for info, and the example of the result.
It should. The script works off the page url, but you can add a background image directly in the splash screen page with css since the page accepts html.
You can simply add this directly to the top of the splash screen page in the admin panel. Just insert your url between the " " marks. You don't have to have the !important tag here since it is directly related to the splash screen page.
<style>
.ow_page_wrap {
background-image: url("your image url here");
background-size: 100% 100%;
background-attachment: fixed;
background-repeat: no-repeat;
}
</style>