I checked this posting, so it was impossible to do it. But it's old posting, and somebody are saying me there is an app which can change wallpaper(lock screen). Is this allowed now?
-- edit --
I'm sorry for my question was unclear. I was asked a programmatic and regular (which can be approved by Apple AppStore) way to change lock-screen wallpaper in my own app.
Not for any hacked, jail-breaken devices.
As far as I'm aware, you would not be able to change the wallpaper for the device through the SDK, or at least not in a way in which Apple would approve of. I suspect allowing this through the SDK could cause problems, apps could obnoxiously add advertisements to the wallpaper, etc.
The only way I can think of around your problem would be to save the target image to the user's camera roll and instruct the user to change it for themselves.
There may be a way to change the wallpaper for a jailbroken device, but this of course violates the iPhone developer program license agreement.