Use webjab or MPT to upload a gif using the same name as the defaults....
To rename wallpaper correctly, make sure you have them as shown below.
Do not put spaces between "Desert" and "Rock", needs to be as shown.
And make sure "gif" is in lowercase.
i870/i880 
Resize to: 176x220
i730 
Resize to: 130 x 130
Other Phones:

Check
Phone Scoop for dispaly size
if you are using Windows XP (home or pro) edition..by default the extensions
are hidden therefore you only see the filename without extension .
To show extensions do the following:
Open My Computer goto
Tools > Folder Options > View
Scroll down till you see
"Hide extensions for known file types"
and make sure it is UNCHECKED
These are the factory names that you have to use to replace the original images: browser_img.gif - replaces the image when you start internet ( will only show the first time you start after powering the phone)
carrier_img.gif - replaces the Nextel image during power up
java_img.gif - replaces java image during power up
pwrdown_img.gif - replaces Nextel done during power down
text_img.gif - replaces the t9 text image during power up
Black&White.gif -downloaded
DesertRock.gif -default stock wallpaper
SailBoats.gif -default stock wallpaper
SunFlower.gif -default stock wallpaper (See attachment below, created with MS paint)