1. Press Switch to image ready (the button which is at the bottom of Tool bar) OR press Ctrl+Shift+M
2. Set the frame 1 of the animation panel to 0.1sec. Duplicate this background 2 times so you have background copy and background copy 2 by pressing Ctrl+j 2 times in Layer Pallete
3. With your move tool selected, use your up arrow key to move the "background copy" layer up till it is just above your canvas.
4. Insert a new frame on your animation panel by clicking the duplicates current frame button on the bottom of the animation panel. Click on frame 2.