The iPhone is a great little machine, no doubt about that. But, it can be a core to change the looks of the iPhoneOS to your likings, especially if you want to change the background of the iPhone’s springboard (check out the end of this article for that).
The first, and definately most easy thing you can do to change your iPhone’s look is to change the background.
This can be done in two ways:
1 Via the settings menu:
Open the settings app, and choose “wallpaper”

This will bring you to an overview of all the images that are available on your iPhone. This means both the wallpaper collection of your iPhone as the photo’s you shot with your iPhone, the images you synced on to your iPhone, or the ones you saved from the web or email.
Select the image you want to use. This will bring you to the “Move and Scale” interface. Here you can zoom in on your picture to crop a part out of it, or to make it fit better. You can use both fingers to move and zoom, just like you’re used to on your iPhone. Adjust your picture until you like it, then tap “Set”, or “Cancel” if you want to select another picture.

2 Via the Photos app:
Open the Photos app, and select the picture you would like to use from your collection.
Tap the outside square icon on the lower left of your picture, and select “Use as Wallpaper”.
This will bring you back in the “Move and Scale” interface, where you can adjust your picture until you like it, then tap “Set Wallpaper”, or “Cancel” if you want to select another picture.

This will result in your chosen picture to be on the background of your iPhone’s lock screen.
Using this method, your wallpaper will only be visible when your iPhone is locked.

If you also want to set a background image on your iPhone’s springboard (that is what the thing is called where you can choose and start your applications), you will need to follow these guidelines: Changing your iPhone’s springboard background.

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