Hello everybody ! 
Here's a little tutorial teaching you how icons can be change on your iPhone/iPod Touch
I. Prison break
Before change anything about GUI on your iPod, you have to jailbreak it!
To jailbreak your iPod, go here http://wikee.iphwn.org/ and find the QuickPwn link.
This will download the QuickPwn app, which will explain you the steps you have to follow in order to do the jailbreaj.
Be careful ! QuickPwn only works with the 1.x and 2.x firmwares of the iPod/iPhone G1, jailbreak isn't available for the G2 ones yet.
II. Cydia
When the jailbreak's done, you can see that QuickPwn installed automatically two apps:
Installer 
will allow you downloading free non offcial app (= not available on App Store).
Cydia as Installer, but principally for themes and themes applications.
Now, forget a little moment Installer, only Cydia interests us. Go on Cydia and download both Winterboard and OpenSSH apps.
Winterboard will help you to skin your iPod (as windowblinds does for WinXP)
OpenSSH will allow you to connect to the root of your iPod through SSH connexion.
When everything's done, connect your iPod/iPhone to a Wifi connewion (normally that's already done, otherwise you couldn't already download Winterboard and OpenSSH ) and connect it to your computer.
III. Opening...
On your computer, run your FTP software, as Filezilla. Fill theses informations :
Host : your iPod/iPhone IP (you can find it in your "Settings" )
User : root
Password : alpine
Port : 22
Normally, you now see the root of your iPod with many directories. If you're not at the root, click on the « .. » folder. When you're at the root, open the Library folder, and then Themes.
The Theme folder contains all the skins that Winterboards handles.
Then download this zip file: Test.theme
This compressed folder contains a Test.theme folder, and will allow you to create a Winterboard theme.
As you can see, it's separed in 3 sub-folders and 4 various files :
- Bundles folder contains the badges which appear on the top-right part of some icons (Mail, Text...)
- Icons folder contains...your icons. Every icons has a special and exact name, because it'll be the name who will determines the icon you want to change. Be sure to wirte with correct letters (and don't forget capital letters).
- UIImages folder containes the litlle indicators of the bottom of your iPod, telling you the number of "screen pages" and the current one.
And 4 files :
- Info.plist Display properties file, like a CSS file in webdesign creation. Modify it to set the text diplays, calendars numbers etc.
- 3 PNGs : Dock, Wallpaper, Statut bar
Put your icon in the Icons folder as I said, connect through SSH to your Ipod, still in the Themes folder. Then drag and drop your Test.theme in the Themes folder, wait, take your iPod, run Winterboard, activate Test, come back to the menu. You'll see a loading animation, and that's done!
Do that for every icon, the Dock, the wallpaper, the statut bar, the badges, and so on...
PS : If you want to change the name of you're skin, reanme Test.theme in NewSkinName.theme
Original topic here (french forum), translated by Troyenne.
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