I tried to replace the file from there but the wallpaper has same quality .

It seems that the lockscreen is actually like a screenshot because if I try to unlock the wallpaper goes back to normal quality .

Here's a video about what I'm talking about . : http://www.youtube.com/watch?v=rIjQHyKhz3U

Why phone manufacturers limit even wallpapers ? I understood they limit ringtones so people will not damage the speaker if they set a louder ringtone , but wallpaper ... wtf ?