Uneven and patterned borders can be created by brushing the edge onto the image. Choose Layer>New Layer, click OK to create a new layer for the edge, and select black as the foreground color. Click the Brush tool; from your paint brush collection, select an interesting brush shape such as a crosshatch shape or stars. Size the brush so that it is big enough to paint the border in one swipe and paint it across all four edges of the image onto the new layer. To get square and even edges, hold the Shift key as you paint to constrain the brush so it travels in a straight line (see Figure 2).
Figure 2 Choosing an interesting brush shape and constraining the painting direction to a straight line gives an interesting edge effect.
This edge color can be inverted by clicking the edge layer and pressing Control+I (Command+I on the Mac). It can also be filled with the current background color (as explained above). You can soften the edge by applying a blur filter to it such as a Gaussian blur or Radial blur or, instead of painting the border in a single step, set the brush opacity to 60% and choose a smaller brush size and paint the border with lots of small swipes to build up the edge (see Figure 3).
Figure 3 Choose a small brush and a 60% opacity, and paint a rough edge onto a new layer using short overlapping strokes.