Bull, even if you don't have internet access on your phone, you can still email to and from, for example, my email addy with Tmobile is
213xxxxxxx@tmomail.net.
To easily find out yours, go to
Create Message
New Text message
Type sfksdlfjl
Select 'OK'
Pick an email address from your phonebook (create one before hand for yourself using your email of choice)
Click 'Add'
Click 'Done'
Click 'Send'
Go to your email on your computer. Reply to the Tmobile message with the JPEG attached.
In your phone, open the reply email, select the JPG image attachment
click 'View'
click 'Store'
click 'Apply as Wallpaper'
Voila, done in 458 steps. It's much easier than it might seem from the above.