Websites where a user can upload images. How does the site owners make it retina ready or scale it according to the retina display?
A retina display is just a screen with very high pixel density (Retina display is Apples version)
To ensure high quality and good user experience:
Consider recommending the user uploads high quality images (but make it clear they don't actually have to)
Do as much as possible yourself to make user submissions retina ready (resize etc.)
Make the transfer from images just taken from different devices to your site nice and easy
Absolutely do not throw back any 'image too large' or similar errors.
This way your users only have to think about their image, displaying, sharing it or whatever else they want to do and don't have to think about any of the internal workings (just as most people just think of cleaning their clothes and not the inner working of their washing machine).
Also you ensure that any given image looks as good as possible in any context because you are prepared to set things up just right for the displays the images are viewed on.