When I buy something, no matter what it is, I consider it mine. In the United States, there are some cell phone companies that don't allow you to customize your phone the way you want like most cell phone companies do. What this means is that they might disallow you to put on a custom ringtone or wallpaper without going through their service to do so, or you'll have to find a way to hack the phone to put the media on yourself. Thankfully, Spring and Palm don't make you go through that trouble to put wallpapers and ringtones on your Palm Pre. In fact, this phone is one of the easiest phones to do it with.
A ringtone is an audio file such as an MP3, WAV, or AAC file that you can set the phone to play when a call comes in. They're generally short and meant to last the length of a standard ringtone, but you can use song fragments or whole songs if you like. For example, if your favorite song is "Love's Theme" by Barry White, you can use that as your ringtone once it's on your phone. Similarly, you can assign different ringtones to different people with the Pre. You can assign "Bad, Bad Leroy Brown" to your friend Leroy, or "Still Alive" to your gaming friend that plays Portal. My ringtone is the Victory Theme from Final Fantasy X.
The easiest way to set your ringtone is to first make sure it plays in a standard audio player such as WinAmp or iTunes. Protected songs may or may not play properly. My .m4p songs wouldn't work as ringtones on the Pre, so making it a .mp3, .m4a, or .wav would be your best bet. Once you verified that the ringtone plays, connect your Pre to your computer with the USB cable that Palm provided with the Pre. Once it's attached, the Pre will ask how you want to connect the Pre to the computer: Charge Only, USB Device, or Media Sync. For this, we'll select "USB Device". The Pre will mount as a drive on your Mac or PC (and will probably work on Linux as well). The screenshot below shows how the Pre mounts on a Mac.
If it doesn't open on its own, open the drive that the Pre mounts as. On the Mac it will have a name, on Windows it will have its own drive letter (eg: F:). Inside there you'll see a folder called "ringtones" which is where the Pre's ringtones are stored. Drag your ringtones in there, and disconnect the Pre from your computer (Windows: Eject Drive, Mac: press the Eject icon). The Pre will now show your ringtones as available to use.
A wallpaper is the image you see when you turn your Pre on. The Pre has several built-in wallpapers but you can create your own. The resolution of the Pre is 320x480, so you can use whatever photo editing tool you want to crop your image to fit that size. Make sure that when you edit your picture, you don't want the lower 1/8th of the picture covered up by the Pre's bottom menu. I used a large wallpaper of Lightning from Final Fantasy XIII as my wallpaper.
You mount your Pre the same way you would for ringtones ("USB Device"), but this time you want to drag your wallpaper into the "wallpapers" folder. When you disconnect the Pre, you go into the "Screen and Lock" app and click on "Change Wallpaper". That's it!