Have you tried to go to settings> wallpaper/ and there select a picture and try to set it as your home screen wall paper? Try first with any of the default wallpapers that came with the iOS. If that doesn't work, then go to your camera roll and select a picture and use it as your wall paper. If none of those working then you will need to root into the device.
yes i have tried that, and every time i reboot or respring the annoying "rick rolled" image appears again. I have used ifile to delete the theme from my iphone, but i do not know how to find this particular image that came with the theme!