If your phone supports external card, the easier way is to put the new wallpaper picture in the external card and plug it into the phone.
Using the setting menu, change the wallpaper setting and choose the picture you would like.
If your phone does not support external card (like my CDM-9900), you can still upload the picture to your phone by sending yourself a picture e-mail. To do so, you send an e-mail with the picture attachement to
yournumber@vzwpix.com. After you receive the message, go to Option and Save the message as Picture. Now, the picture is in your media gallery or My Pix section.
Using the setting menu, change the wallpaper setting and choose the picture you would like.
(Receiving picture message will cost you $0.25 per message. Still cheaper than purchasing it from Verizon)