I am not entirely sure about your question so here are a few notes which may be of use.
I am under the impression that most computer, mobile, phablet and tablet screens have a maximum screen resolution of 72 ppi. Using a higher ppi will increase the file size (slower file loading) and will not display any better quality.
I am not sure about the latest Retina displays, no doubt some other poster will enlighten us ignorant folk
As far as displaying larger image files on a small screen I opt for the following CSS
<img src='very-big-image-1024x888.jpg' style='width:98%; max-width:1024px; border:0' alt='very big image scaled to suit all screen sizes' />
- for screens smaller than the maximum image width a correctly scaled down image will display.
- for screens larger than the maximum image width the complete image will be displayed without scaling.
- 1024px corresponds to the maximum image width, larger values will display proportionately but lose quality.