image editor layer filters. Start by creating a new layer and fill it with a solid color using the Paintbucket tool. The color of the fill does not matter since it won't be visible in the end product. Next add a Glow effect from the layer filters panel. Enable both the Inner and Knockout parameters, set the blur x and blur y somewhere around 20, and set the Color to black. This will create a nice shading around the edges of your image, and at anytime you can adjust any of the parameters to change the effect. Play with the blur values to expand/contract the vignette, alter the color to give it a different vibe, or even change the quality to fine tune the effect.