1. Open your header for your website, in this tutorial we just used a background.
2. Click Layer > New > Layer from background - which basically makes the background unlocked
3. Next, hit Image > Canvas size, and make your height twice as high as it was.
4. Grab the polygonal maqruee tool, and make a triangular selection (hold shift whilst doing the diagnol bit, as it gives you an exact 45* angle)
5. Hit delete, to get rid of the selection
6. Hold CTRL and press D to deselect, and with the polygonal marquee tool, make a bigger selection (see image) quite a way up the side and the bottom.
7. Click Selection > Feather - and feather it about... 30px
8. Next, Filter > Pixelate > Mosaic - and give it a cell size of 5 - hit OK
9. Click Filter > Pixelate > Mosaic, another 2 times
10. Filter > Sharpen
11. Set your background colour to a colour, that you would have if the image was blank. for our example, we might choose light blue, or white.
12. Click Image > Canvas size - and make the height a half of waht it now is.
13. Layer > New > Background from layer (which should give you that background colour)
14. File > Save
