I would suggest this:
- For your image "http://www.akashawacha.co.uk/wp-content/uploads/2012/08/bg-body1.jpg", redo it with just the feather/mask/etc. images without the pattern background and save it as a transparent ".png"
- I'm assuming that the pink/blue pattern is a small repeating image that you used to create the background when you made the complete image.
Upload both new images to your WP Media.
In your child theme's CSS:
- For the small pink/blue repeating image, which will be behind the feather/mask image:
background:transparent url("http://www.akashawacha.co.uk/wp-content/uploads/2012/08/your_repeating_pink-blue_image.png") fixed center top repeat;
- For the feather/mask/etc. image:
background:transparent url("http://www.akashawacha.co.uk/wp-content/uploads/2012/08/your_feather-mask_image.png") fixed center top repeat-y;
In having the feather/mask image repeat on the y-axis (vertically), you will see it if the browser window is higher than your initial image.