This example shows, how to make a UIView or UIImageView, rounded with some radius like this:

Objective-C

someImageView.layer.cornerRadius = CGRectGetHeight(someImageView.frame) / 2;
someImageView.clipsToBounds = YES;

Swift

someImageView.layer.cornerRadius = someImageView.frame.height/2
// this should alleviate the performance hit that adding transparency may cause - see <http://stackoverflow.com/a/6254531/189804>
// Be sure to check scrolling performance with Instruments if you take this approach.
someImageView.layer.shouldRasterize = true
someImageView.clipsToBounds = true // All parts of the image that are outside its bounds (the frame) are cut out (makes the rounded corners visible)

It is suggested that if you use autolayout that you put the someImageView.layer.cornerRadius code in viewDidLayoutSubviews. This will allow the image’s cornerRadius to update if the image changes size.

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    someImageView.layer.cornerRadius = someImageView.frame.size.width/2
    someImageView.layer.masksToBounds = true
}