Here is a revised response with the following modifications:
Centered: To centre the text within the generated UIImage, draw(at:withAttributes:) was used instead of draw(in:withAttributes:).
Correct Size: Used size(withAttributes:) to produce a UIImage with a size that corresponds to the font's true size.
Comments: Added remarks to aid comprehension
Swift 5
import UIKit
extension String {
    func textToImage() -> UIImage? {
        let nsString = (self as NSString)
        let font = UIFont.systemFont(ofSize: 1024) // you can change your font size here
        let stringAttributes = [NSAttributedString.Key.font: font]
        let imageSize = nsString.size(withAttributes: stringAttributes)
        UIGraphicsBeginImageContextWithOptions(imageSize, false, 0) //  begin image context
        UIColor.clear.set() // clear background
        UIRectFill(CGRect(origin: CGPoint(), size: imageSize)) // set rect size
        nsString.draw(at: CGPoint.zero, withAttributes: stringAttributes) // draw text within rect
        let image = UIGraphicsGetImageFromCurrentImageContext() // create image from context
        UIGraphicsEndImageContext() //  end image context
        return image ?? UIImage()
    }
}
Swift 3.2
import UIKit
extension String {
    func textToImage() -> UIImage? {
        let nsString = (self as NSString)
        let font = UIFont.systemFont(ofSize: 1024) // you can change your font size here
        let stringAttributes = [NSFontAttributeName: font]
        let imageSize = nsString.size(attributes: stringAttributes)
        UIGraphicsBeginImageContextWithOptions(imageSize, false, 0) //  begin image context
        UIColor.clear.set() // clear background
        UIRectFill(CGRect(origin: CGPoint(), size: imageSize)) // set rect size
        nsString.draw(at: CGPoint.zero, withAttributes: stringAttributes) // draw text within rect
        let image = UIGraphicsGetImageFromCurrentImageContext() // create image from context
        UIGraphicsEndImageContext() //  end image context
        return image ?? UIImage()
    }
}