This is a category on UIImage.

gistfile1.m

- (UIImage *)rxScalePropertionallyToMaximumSize:(CGSize)maxSize blendMode:(CGBlendMode)blendMode alpha:(CGFloat)alpha scale:(CGFloat)scale {
    
    if ((maxSize.width < 1.0e-15) || (maxSize.height < 1.0e-15)) return nil;
    
	CGSize imageSize = self.size;
    CGSize scaleFactors = {
        .width = maxSize.width / imageSize.width,
        .height = maxSize.height / imageSize.height,
    };
    
    CGFloat scaleFactor = MIN(scaleFactors.width, scaleFactors.height);
    
    CGSize newSize = {
        .width = imageSize.width * scaleFactor,
        .height = imageSize.height * scaleFactor
    };
    
    CGRect imageFrame = (CGRect){
        .origin = CGPointZero,
        .size = newSize
    };
    
    UIGraphicsBeginImageContextWithOptions(newSize, YES, scale);
    [self drawInRect:imageFrame blendMode:blendMode alpha:alpha];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return newImage;
}

This is a category on UIImage. It's intended to take a photo from the camera, image library or camera roll and shrink it. (The input size is smaller than the original size.)

But I've been having a lot of problems getting it to work with all orientations and sources.

I think this version will work.