Is there a UIView resize event?

In a UIView subclass, property observers can be used:

override var bounds: CGRect {
    didSet {
        // ...
    }
}

Without subclassing, key-value observation with smart key-paths will do:

var boundsObservation: NSKeyValueObservation?

func beginObservingBounds() {
    boundsObservation = observe(\.bounds) { capturedSelf, _ in
        // ...
    }
}

As Uli commented below, the proper way to do it is override layoutSubviews and layout the imageViews there.

If, for some reason, you can't subclass and override layoutSubviews, observing bounds should work, even when being kind of dirty. Even worse, there is a risk with observing - Apple does not guarantee KVO works on UIKit classes. Read the discussion with Apple engineer here: When does an associated object get released?

original answer:

You can use key-value observing:

[yourView addObserver:self forKeyPath:@"bounds" options:0 context:nil];

and implement:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (object == yourView && [keyPath isEqualToString:@"bounds"]) {
        // do your stuff, or better schedule to run later using performSelector:withObject:afterDuration:
    }
}