How do I force a Retina display to use CSS Pixels and ignore Device Pixels

I have limited CSS skills so I am working outside my comfort zone.

I am trying to make a smaller screen eg iphone display a different background (with overlaid images) than appears on a larger device. Image content is different, not just a smaller version of the larger image.

This works fine when I set screen width to eg 320px on desktop, but devices with retina display are loading larger images.

Is there a global method to force the device to ignore the device pixels (preferably for all images and text) so eg an iPhone3 and iPhone4 (retina) will both load the same 320px wide image + other images.

If no global command what can I do with each background and other images get the outcome I want?

I am not concerned about any pixelation on retina diaplsy.

Thanks ColinK