considering that they are all square, I'm going to say that they are 1024x1024 so to retain the image quality, even when rotated.They're 1024x768. A great deal of effort was probably put into downsizing them from a higher resolution to make them look as good as possible on an iPad.
They don't need to be "above" 1024x1024 for best full resolution but be exactly 1024x1024. If above (or below) 1024x1024 they would need to be downscaled (or upscaled).This.
The device's resolution is 1024x768, but wallpapers need to be above 1024x1024 to have the same detail when rotated.