1. Original Image | 2. Hue | 3. Saturation | 4. Lightness
5. Gaussian Blur | 6. Negative | 7. Average of 4,6 | 8. Add Contrast
First I took the original image and split it into HSL channels (images 1 through 4.) As you can see, this image is a pretty constant blue color, so the "hue" and "saturation" channels are basically one solid color.
(Actually I decided to make them solid color because the only details visible were artifacts of JPEG compression - don't use JPEG as a source format if you want the best quality textures.)
Then I took the Lightness channel and applied a "Guassian Blur" to it. This takes some experimentation. The blur radius should be just barely large enough that it does not include any of the details you want to keep. In this case there are some large bumps with a radius of about 10 that I didn't want to remove, so I used a radius of 20. (See image 5.)
Next I made the blurred image negative (image 6), and then "averaged" it with the original Lightness (image 4) by using Paint Shop Pro's "image arithmatic" feature (selecting two images, "Add" and setting the "divisor" to 2 will give you the "average" of the two images.)
The result is image 7, but that image has a lot less contrast than the original. This was fixed by increasing the contrast by 50 points. This is another step you might want to play around with before making a final decision. The best contrast is probably whatever gets you closest to the original look of the image.
Finally I combined the Hue and Saturation channels with the new image as Lightness (Paint Shop Pro required me to make the new image grayscale before it could be selected as a channel.) And the result, at full scale, is the background of this page.