These can be added in many ways depending on what software you have. If you post what software you use then I'm sure we can explain how to achieve the effect.
__________________ My Kit: A camera and some lenses
In Lightroom, go to the Develop module. On the right-hand tab, scroll to the bottom and you'll see "vignette" and "post crop". In the "post crop" bit, slide the top slider to the left. Job done. Huzzah!