For those who are using Photoshop Elements, you actually have a really quick way to add a vignette to an image via the Guided Edits panel. All you need to do is click on 'Guided' from the top menu and ...

To the righthand side, you'll find a list of tools which includes the Vignette slider (and associated tools). To add a Vignette, simply just drag the slider left or right (right will give you a dark ...