I found tutorial from Matthijs Hollemans on scaling graphics designed for the retina display on the iPhone 4 and iPod touch 4g for the lower resolution on older versions of the iPhone and iPod touch, and this is also useful if you will be using your graphics on other phones with a lower resolution.
The easy way out is of course just to load your images in an image editor such as Photoshop, and lower the resolution automatically using the bicubic filter. In Matthijs tutorial he explains that this does not produce optimal results, and provides a simple command-line tool for producing better looking scaled down graphics.
Matthijs also provides a few other tips explaining how to create the best possible looking scaled down graphics.
You can find Matthijs’ tutorial and his Shrink PNG tool on his website here:
http://www.hollance.com/2011/04/drawing-retina-graphics/
If you want to make sure that every detail looks as good as it can when the graphics are displayed on a smaller resolution screen then you will want to check it out.
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