Make Your Own: iPad Wallpaper
The screen is 1024 pixels (on the long side) and 768 pixels (on the short side), however your image needs to be a square shape to be able to accommodate both of these sizes. 768 x 768 pixels in the center are always visible but as I said before: when your screen rotates different areas of the image will be visible depending on the view as shown below:
Never again will you need to use one of the boring ol generic backgrounds that come with your device. Everyone is going to be so jealous of your amazing computer skills!