If you have the ability to upload Wallpaper files to your iPhone or your iPod Touch then you need to know the proper size. The default wallpaper files are located in the /Library/Wallpaper directory and include the default wallpaper files and the thumbnails for those wallpapers. To get the iPhone wallpaper to display correctly you need the proper sizes.
iPhone & Touch Wallpaper Size: 320 x 480 pixels
iPhone & Touch Wallpaper Thumbnail Size: 75 x 75 pixels
So first create the main wallpaper file at the proper size above and save it as a .PNG file. Next create the thumbnail file in the correct size listed above and save as a .PNG file. Then secure copy the files to your iPhone or iPod Touch to the /Library/Wallpaper directory. Once these steps are completed you can set the new file as your wallpaper by clicking on the Settings icon, selecting General, selecting Wallpaper, and then choosing the new file.
**NOTE: You are not required to have a thumbnail file though you may want to create one so it displays properly in the list of wallpaper files.