Jagged Edges

Start with your image opened in GIMP. :-) 1. Open your Channels Dialog and create a new channel (it should initially be filled with black). 2. Select All. 3. Shrink your selection by the thickness of your desired border. 4. Use the Rectangle Select Tool to expand the left and right sides of the selection out to the image edges. 5. Bucket fill the channel with white (you should have a white channel with black strips along the top and bottom). 6. Select None. 7. Apply the "Filters->Distorts->Ripple" with a Wavetype of "Sawtooth" and an Orientation of "Vertical" to this channel. Adjust the other parameters (Period, Phase, and Amplitude) to achieve a nice symmetrical result of the proper dimensions. Repeat steps 1 through 7 only this time expand the rectangular selection (in Step 4) so that you will end up with black strips along the side edges and change the Orientation to Vertical when applying the filter in Step 7. Activate one of the channels and then click on the red square next to the trash can in the row of buttons along the bottom of the dialog box (this will load the channel as a selection). Activate the other channel and hold down the CTRL & SHIFT keys as you click on the red button (this intersects the channel with the selection). These two steps have the effect of combining your horizontally and vertically rippled channels into the selection. Go to your Layers Dialog and activate your image layer. Perform a "Layer->Mask->Add Layer Mask" and where it says "Initialize Layer Mask To:" click on the button labeled Selection. If all has gone well, you should have the desired result (and it probably took you less time than it took to write this).