First, if you decide to jailbreak your iPhone or iPod touch and Apple finds out about it, you will no longer have a warranty with Apple (because Apple can't varify that the jailbreak software is safe for the iPhone and because Apple didn't create the jailbreak software). So, if you jailbreak and something messes up you iPhone/iPod touch, you will be responsible.
To jailbreak quickly and easily, I recommend using ZiPhone.
Go to
http://www.ziphone.org
Click the link on the top of the screen. It should say something like "Download Main"
Then click the one that applies to you (windows or mac).
Then go into the application and click "Jailbreak" (under "iPhone/iPod touch").
That's it. It's very simple. Then you can go into the new application on your homescreen called "Installer" and find SummerBoard or FreeBoard (and also Customize if you want) and they will allow you to add an image in your backround, as well as a lot of other functions.