Choose Image Resolution for iPad and iPhone
The wow factor of the iPhone and iPad is that they’re well suited to displaying beautiful, lifelike images. For the web designer, displaying images correctly on the iPad and iPhone is an undertaking that requires at least a working knowledge of graphics standards, careful planning, and an appreciation of the limitations of mobile displays.
When you save images for the web, most web designers recommend that you save them at a resolution of 72 pixels per inch (better known as ppi). Most computer monitors display no more than 72 ppi, so any resolution higher than that is wasted on the web because it requires your visitors to download more pixels than they can see.
However, if you want to print an image, you want all the pixels you can get, usually at least 200 ppi or higher, which is why most images you see on the web look terrible if you try to print them in a large size.
When you’re designing for the high-resolution iPhone and iPad screens, you may want to increase the number of pixels per inch to better match the resolution of these devices. Remember that the iPad displays 132 ppi; the iPhone 4 displays 326 ppi.
While it’s probably not a good idea to create images with a 326 ppi resolution for use on the Internet, if you want your images to look their best and you don’t mind sacrificing a little download time, you may be happier with the results if you set the resolution of your images to 92 ppi, or even 132 ppi.