your app should retrieved those image in retina size. to get the non-retina size, you can scale it manually then save it. here the example code :
UIImage *image = [UIImage imageNamed:@"yourRetinaImage@2x.png"];
// set non-retina size from current image
CGSize size = CGSizeMake(image.size.width / 2., image.size.height / 2.);
/** scale the image */
UIGraphicsBeginImageContext(size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextTranslateCTM(context, 0.0, size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, size.width, size.height), image.CGImage);
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
/** save scaled image */
NSString *basePath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
// save with same name but without suffix "@2x"
NSString *filePath = [NSString stringWithFormat:@"%@/%@", basePath, @"yourRetinaImage"];
@try {
[UIImagePNGRepresentation(scaledImage) writeToFile:filePath options:NSAtomicWrite error:nil];
} @catch (NSException *exception) {
NSLog(@"error while saving non-retina image with exception %@", exception);
}