GIMP/Add a Shine to Buttons
What the Effect Looks Like
Design the Button
Add a New Layer for the Shine
Create a new layer (call it shine or something similar), then select the whole region of your button (If you created it on a new layer you can right click on the layer and select “Alpha to Selection”), next shrink the selection (Image>Select>Shrink), how much you want to shrink it depends on the size of the image, for mine I used 6px, then fill the selection with white.
Turn the Ugly Spot into a Shine
Then go to the gradient tool and select linear white to black gradient, (make sure you are editing the layer mask-- it should have a white border around it in the layers dialogue), draw a gradient from wherever you want the shine to start (Top-left corner in the example) to about the middle, feel free to experiment.