I'm not certain how upgrades may impact it, but to change the background image for login is relatively simple in IIS.
You can find the background login in the directory C:\Program Files\NetApp\SnapCenter WebApp\Content\img\LoginBackdrop.jpg
Simply download the desired image as a jpeg file and rename the original image file and new image file respectively, and place the new image file into the directory.
I would imagine most upgrades would not impact this, however it is relatively easy to repeat following an upgrade.
