You can do this by inserting a new subview to your TabBar.
Please check out this answer:
// Add background color to middle tabBarItem
let itemIndex = 2
let bgColor = UIColor(red: 0.08, green: 0.726, blue: 0.702, alpha: 1.0)
let itemWidth = tabBar.frame.width / CGFloat(tabBar.items!.count)
let bgView = UIView(frame: CGRectMake(itemWidth * itemIndex, 0, itemWidth, tabBar.frame.height))
bgView.backgroundColor = bgColor
tabBar.insertSubview(bgView, atIndex: 0)
Hope it helps.
Edit:
If you want to change the background image
rather than background color
, all you have to do is change the line :
bgView.backgroundColor = bgColor
to imageView with image as background, then add it as a subview. It may look like this:
backgroundView = UIImageView(image: UIImage(named: "tabBarImage"))
bgView.addSubview(backgroundView)