I was struggling with this for about a minute yesterday. So I thought I would save others the trouble. If you're trying to change the background you have to follow this :
Note: Starting with 4.2.1 firmware these graphics have been renamed.
FolderIconBG.png becomes FolderIconBG~iphone.png
FolderIconBG@2x.png becomes FolderIconBG@2x~iphone.png
FolderSwitcherBG.png becomes FolderSwitcherBG~iphone.png
FolderSwitcherBG@2x.png becomes FolderSwitcherBG@2x~iphone.png
In order to enable these graphics with 4.2.1. firmware and later, you need to either rename or delete the following files on your iPhone found at: /var/mobile/Library/caches
com.apple.SpringBoard.folderSwitcherLinen.0
com.apple.SpringBoard.folderSwitcherLinen.1
com.apple.SpringBoard.folderSwitcherLinen.2
com.apple.SpringBoard.folderSwitcherLinen.3
I don't believe in deleting files on a live file system. I renamed my files to 4, 5, 6, and 7.
My tip is that after you've resprung and you try to change the background it doesn't work. This is because the OS recreates the files you just renamed so it locks the new image. So you have to rename the files each time.
Note: Starting with 4.2.1 firmware these graphics have been renamed.
FolderIconBG.png becomes FolderIconBG~iphone.png
FolderIconBG@2x.png becomes FolderIconBG@2x~iphone.png
FolderSwitcherBG.png becomes FolderSwitcherBG~iphone.png
FolderSwitcherBG@2x.png becomes FolderSwitcherBG@2x~iphone.png
In order to enable these graphics with 4.2.1. firmware and later, you need to either rename or delete the following files on your iPhone found at: /var/mobile/Library/caches
com.apple.SpringBoard.folderSwitcherLinen.0
com.apple.SpringBoard.folderSwitcherLinen.1
com.apple.SpringBoard.folderSwitcherLinen.2
com.apple.SpringBoard.folderSwitcherLinen.3
I don't believe in deleting files on a live file system. I renamed my files to 4, 5, 6, and 7.
My tip is that after you've resprung and you try to change the background it doesn't work. This is because the OS recreates the files you just renamed so it locks the new image. So you have to rename the files each time.