iPad wallpaper compression …

Did myself a few iPad wallpapers with some wireframe Spook renders i like.
But i have a problem… Why do some wallpapers, these in particular , appear highly compressed (like saved at low quality .jpg format) when they are set as wallpaper on the iPad ?!
And this even if i save them in Png or good quality Jpg. they look perfect in the photo album or other image viewing apps on the iPad, but as soon as they are set as wallpaper, it’s horrible :(
I’ve only found one page talking about this problem , here :
« does-iphone-compress-images-you-set-them-wallpaper. »
But it doesn’t give any solution… moreover this doesn’t seems to happen with every images . So i’ll really like to know why…
Here a zip with the high quality images, 5.4 Mb :
Link

Here the reduced images :