Downloading normal image Vs retina device image (2x)

Question

When we need to download an image from some URL and show it on two kinds of devices -- Retina (with 2x image) and regular device -- Should we have two different image URLs to ha

Answer 1

You can check if the device has a high-resolution retina display and based on that download a different image. Don't bother for photos and stuff that you'd scale anyway for interface size.
You can create the scaled version of the downloaded image with

UIImage *image = //download...
image=[UIImage imageWithCGImage:[image CGImage] scale:2.0 orientation:UIImageOrientationUp];

Keep in mind that a scaled 100x100 image will become a 50x50 points image (with 2.0 scale).

Check first if you have a retina display

BOOL retina = NO;
if([[UIScreen mainScreen] respondsToSelector:@selector(scale)])
    retina = [[UIScreen mainScreen] scale] == 2.0 ? YES : NO;

Answer 2

CGFloat screenScale = [UIScreen mainScreen].
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:myUrl] scale:screenScale];