adding black overlay with 0.3 opacity over UIImageView

Something like this?

UIView *overlay = [[UIView alloc] initWithFrame:CGRectMake(0, 0, myImageView.frame.size.width, myImageView.frame.size.height / 2)];
[overlay setBackgroundColor:[UIColor colorWithRed:0 green:0 blue:0 alpha:0.3]];
[myImageView addSubview:overlay];

This is similar to MDT's answer except using CALayer properties:

UIView *blackOverlay = [[UIView alloc] initWithFrame: imageView.frame];
blackOverlay.layer.backgroundColor = [[UIColor blackColor] CGColor];
blackOverlay.layer.opacity = 0.3f;
[self.view addSubview: blackOverlay];

The MDT answer is correct. This is just another way to use a CAGradientLayer beside of UIView . I think it will make what you want with more graphical options.

first you should add

#import <QuartzCore/QuartzCore.h>

to your ViewController.m and any place that you want to add this overlay to your UIImage use:

CAGradientLayer *gradientLayer = [CAGradientLayer layer];
gradientLayer.frame = myImageView.layer.bounds;

gradientLayer.colors = [NSArray arrayWithObjects:
                        (id)[UIColor colorWithWhite:0.9f alpha:0.7f].CGColor,
                        (id)[UIColor colorWithWhite:0.0f alpha:0.3f].CGColor,
                        nil];

gradientLayer.locations = [NSArray arrayWithObjects:
                           [NSNumber numberWithFloat:0.0f],
                           [NSNumber numberWithFloat:0.5f],
                           nil];

//If you want to have a border for this layer also
gradientLayer.borderColor = [UIColor colorWithWhite:1.0f alpha:1.0f].CGColor;
gradientLayer.borderWidth = 1;
[myImageView.layer addSublayer:gradientLayer];

I hope this will help you make it

Thanks to Mick MacCallum

Swift 3 Version

let overlay: UIView = UIView(frame: CGRect(x: 0, y: 0, width: cell.imageView.frame.size.width, height: cell.imageView.frame.size.height))
overlay.backgroundColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.1)
cell.imageView.addSubview(overlay)

I have used for Swift 2.2

let overlay: UIView = UIView(frame: CGRectMake(0, 0, cell.imageView.frame.size.width, cell.imageView.frame.size.height))
overlay.backgroundColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.1)
cell.imageView.addSubview(overlay)