The cell probably has an IBOutlet called checkBoxOutlet and your's does not, that's why. You need to create one and link it to the checkbox in your view.
...or maybe you named your outlet checkBoxOutline, in that case you should write cell. checkBoxOutline.
You need to create an UIImage and pass it as an argument to setBackgroundImage(). Also, this isn't related to Swift, but UIKit, so it would be best if you post any further queries in the developer forums.