About customizing and resizing UITabBarController icons

I want UITabBarController to display custom icons on tabs

This approach is not correct.The approach is to the View Controller.
On the Storyboard, click the icon on the View Controller tab bar to see the Attributed Inspector shown below.

Set the System Item to ImCustom 」 and specify the Image as any image.The image does not have to be a "line drawing" and can be a bitmap image of 32 pixels or less on each side.

To specify an icon when you add a View Controller to a Tabbar Controller in a program.(This sample code should be written to any View Controller class.)

let thirdViewController=UIViewController()
thirdViewController.view.backgroundColor=UIColor.yellowColor() // Customize View Controller
thirdViewController.tabBarItem.image=UIImage(named: "third.png")// The filename is optional.
var viewcontrollers = self.tabBarController ?.viewControllers
viewcontrollers?append(thirdViewController)
self.tabBarController?.setViewControllers(viewcontrollers!, animated:true)

As you can see, the tab bar icon is set to a UIViewController, not a UITabBarController.