if you have iFile or are familiar with SSH here is a step by step:
System/Library/CoreServices/SpringBoard.app
find the following files:
Code:
WallpaperGradientPortraitBottom@2x~iphone.png
WallpaperGradientPortraitBottom~iphone.png
and rename by putting a 1 in front ... respring
Code:
1WallpaperGradientPortraitBottom@2x~iphone.png
1WallpaperGradientPortraitBottom~iphone.png
to revert back, simply remove the 1 ... respring