The DPI does not really come into play as that is mostly just for printing purposes.
You should leave all of your DPI settings at 72dpi, which is the standard for monitors.
What you should be concerned with are the pixel dimensions of your images.
The iPhone4 retina display pixel dimensions are 960x640
The iPad screen dimensions are 1024x768
Older iOS devices are 480x320
Android devices all vary.
(those dimensions are for width x height for landscape, switch them for portrait mode)
In Photoshop, or whatever image editing program you are using, make your graphics match the target device you are working for.
Corona has some nice features for content scaling, and there is a recent blog post discussing designing for different screen sizes.
It is definitely a bit of a challenge to design for multiple devices. The iPad for example, has roughly the same amount of pixels as the iPhone4 - but the sizes of the devices are vastly different. The iPhone4 packs all those pixels really tightly.
So in that case, you cannot simply make your graphics for the iPad and directly port them over to the iPhone4. All of your buttons that are the correct size on the iPad, would be too tiny to interact with on the iPhone4.
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