P8000 Lock Screen wallpaper problem

I have the latest ROM on my P8000 but I see a problem with replacing the lock screen wallpaper.

1. It stretches the picture which is not pretty especially if you put a person there.

2. It lags a lot when you put a custom image. If you choose one from the list that provides you in the menu then it's fine.

Has anyone figure out where the stock images are to replace one of those as a lockscreen picture? Is there a fix about this?