I want to display an image to my users which is larger than the screen area and will overscroll to let the users drag it around and zoom in.
Do I need different images with different dpi for iPhone 4 users because of the higher resolution?
Or can I just set the zoom level differently depending on device?
Yes, you need to use different images. iOS 4 will automatically detect a higher-res image if you name it with a @2x
suffix. e.g. image.png => [email protected]
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