Fade/dissolve when changing UIImageView's image

It can be much simpler using the new block-based, UIKit animation methods.

Suppose the following code is in the view controller, and the UIImageView you want to cross-dissolve is a subview of self.view addressable via the property self.imageView Then all you need is:

UIImage * toImage = [UIImage imageNamed:@"myname.png"];
[UIView transitionWithView:self.imageView
        duration:5.0f
        options:UIViewAnimationOptionTransitionCrossDissolve
        animations:^{
  self.imageView.image = toImage;
} completion:nil]

Done.

And to do it in Swift, it's like so:

let toImage = UIImage(named:"myname.png")
UIView.transitionWithView(self.imageView,
                          duration:5,
                          options: UIViewAnimationOptions.TransitionCrossDissolve, animations: { self.imageView.image = toImage }, completion: nil)

Swift 3, 4 & 5

let toImage = UIImage(named:"myname.png")
UIView.transition(with: self.imageView,
                  duration: 0.3,
                  options: .transitionCrossDissolve,
                  animations: { self.imageView.image = toImage },
                  completion: nil)

Edit: there is a better solution from @algal below.

Another way to do this is by using predefined CAAnimation transitions:

CATransition *transition = [CATransition animation];
transition.duration = 0.25;
transition.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
transition.type = kCATransitionFade;
transition.delegate = self;
[self.view.layer addAnimation:transition forKey:nil];
view1.hidden = YES;
view2.hidden = NO;

See the View Transitions example project from Apple: https://developer.apple.com/library/content/samplecode/ViewTransitions/Introduction/Intro.html#//apple_ref/doc/uid/DTS40007411

For Swift 3.0.1 :

UIView.transition(with: self.imageView,
              duration:0.5,
              options: .transitionCrossDissolve,
              animations: { self.imageView.image = newImage },
              completion: nil)

Reference: https://gist.github.com/licvido/bc22343cacfa3a8ccf88