The initial wallpaper on the screen of the P168 is the same as the Iphone one (what a coincidence :-) )
To change it we need to go to the main menu, then the fourth icon in the left column ("System") and there you choose "Settings".
After that go to "Phone Setup" and then select "Display Characteristic" and choose the option 1 ("Wallpaper").
You've got two option now, either you choose one of the system wallpapers with the option 1 or you select the option 2 ("User Define") and enter the Memory Card.
In the folder "Images" there are some pre-charged images that come with the phone and we can upload our own to this folder to use the ones we like.
The size in this case should be 240x320.

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