It has an extremely easy-to-use GUI (Graphical User Interface, for those who don't know). It uses any 256kb or less image, and stretches it to fit your screen. You can either pre select an image of that size, or you can let TweaksLogon adjust the image. This makes a copy of it for it to set as the background, and it also backs up every image used as a background.

Another good feature let's you preview the login screen after you set the image.

Link<-Download Link
Link<-Original Source

This is not an illegal use of software. You can do this by other means, this is just the simplest way of doing it.

Link<- Link to MakeUseOf article about another method of changing the image. Also mentions TweaksLogon at the end.

Enjoy!