How to Change the Text Message Background on the iPhone 3G
One unfortunate feature of the iPhone 3G is that it comes with only one default text message background. Unless you have a jailbroken phone, you won't be able to change this background. Jailbreaking your phone is quite easy, however, and once you've done so, you'll have Cydia installed on your phone, an application that allows users to take advantage of applications and features outside the Apple system.
Jailbreaking your iPhone 3G
Open the internet browser on your iPhone 3G and navigate to jailbreakme.com
Slide the button once you arrive at jailbreakme.com. The slide button will be located at the bottom of the page beside "slide to jailbreak."
Wait for Cydia to be installed and tap on "OK" when the message "Cydia has been added to the home screen" appears.
Changing the Text Message Background
Open the Cydia app on your iPhone 3G.
Tap on the "Search" icon in the bottom right corner, search for "SMS/Desktop Background" and install it.
Tap on the settings icon of your iPhone, tap on "Wallpaper" and select one of the wallpapers that are displayed.
Tap on the upper left "i" icon to change the text message background to the wallpaper you selected.