This is sadly not an easy task as it seems, The theme itself can be found in Jeff's Github. That repo is a little dated, bodhi is actually using the version found in Štefan's repo. The Splash screen on login is actually defined in the default.edc file in the group e/init/splash. If you examine the code there you will see the image displayed is actually made up of a series of other images. All found in the root folder of the theme itself.
So it is not a single image. You could write an efl program to assemble it and display the result, much like moksha does when it loads and displays the splash.
EDIT: It occurs to me that if all you want is that image to use as a Moksha background then you can create an edc file for a background using those same images and compile it for use by moksha. Perhaps easier, depends upon your point of view I suppose.
Beautiful background isn't it. Makes me think I should look over all the splash screens in all our themes. Like I have time for all that, lol. Anyways glad to have help and much thanks for the rather generous Donation