The iPad has landed and photographers across the USA are finding it to be an amazing device for sharing photos. But because the iPad senses which direction you’re holding it, and automatically adjusts the perspective, i.e., portrait or landscape, there is some confusion as to what size your images should be for proper iPad display.
If you know that you have only horizontal images and you know that those images would never be displayed in portrait orientation, you could simply size your images to be 1024 on the horizontal side by 768 on the portrait side. But you probably don’t only have landscape orientation in every photo and you sure can’t guarantee that someone won’t turn the iPad around and view the images in a format you don’t approve.
So the answer is to compromise. My iPad application Avian Wallpaper uses images that are 1024×1024 pixels in size. This seems to create the best result whether viewing the images in landscape or portrait mode.
1024×1024 is the smallest size that will allow the image to be displayed without upsizing on the display. Since the screen is 1024×768, in landscape mode the top of the image is clipped out, and in portrait the sides are clipped. In both cases the longest dimension is 1024 pixels. The image could be bigger than that, but then you would either be downsizing it or having the user adjust its position when they set it as wallpaper.