It`s nothing to do with SP2 pop-up blocker and if it is when the screen is still black (the POST screen) its prior to Windows even booting and is therefore not even connected with Windows. Some motherboards such as Asus do have the ability in the BIOS settings to show a wallpaper whilst POSTing which is enabled/disabled in the BIOS, Some of these can even be to the users design.The only alternative to this would be a Boot virus but they are usually more destructive than just an annoying picture.