iPad wallpapers should be 1024 x 1024 pixels in size as there is no right or wrong way to hold and iPad. When you change the orientation from landscape to portrait the wallpaper changes orientation and crops from it from two sides.
So, the perfect wallpaper should be 1024 x 1024 pixels with the object centred in a 768 x 768 pixel area.
On the attached image you will see a perfect iPad wallpaper. The subject, Megan Fox, is centred in a 768 x 768 area and whether in landscape or portrait she will be totally seen. Also indicated on the image are the areas in each corner that will never been seen.