Overview
The iPhone has a 320x480 screen. The initial screen when the phone is locked has a background that can easily be changed from the settings menu. To import pictures from a mac, create a folder in iPhoto called "iPhone Wallpapers" or something similar. Copy the backgrounds you would like into that folder. Then in iTunes tell it to sync that folder with your iPhone. It will then create an album on the iPhone you can use to set the wallpapers.
It is also worth noting you don't have to create a background before using it since you can crop and zoom in on any photo you have in the iPhone. The problem comes up when you are zooming in on a photo. iTunes "optimizes" the photos for the iPhone, meaning it reduces the quality. So when you zoom in on an image things can start to look grainy. If you create an image initially at the correct resolution then you will have as good a quality picture as possible.
Keep in mind when creating your backgrounds that the top and bottom portions of the screen will have information like the time or the slider. I've seen several backgrounds that take advantage of the full screen. Although they look good on the computer once you import them into the iPhone you'll see someone's head chopped off or some other issue. Just try to imagine a slightly elongated and slightly lowered from center box and make sure the main subject of the image is in there. You can always just import the full image and resize it. Once it looks how you like you can then duplicate the settings in your photo editor.
If you are looking for free photo editors, I would recommend Paint.Net for windows or Seashore for os x.
Backgrounds
Here are backgrounds I have made. All the pictures on here are created by me and are released under a Creative Commons Attribution-NonCommercial-ShareAlike License. It would be nice if you used one in some open source project that you let me know but isn't required.
Because there are more picture on here, these are reduced in size and quality to improve loading time. Click on the image to view the full version to save.
If you have javascript enabled it will "zoom in" to the full sized image. You can then right click and save the image as usual
 "Mahuri" by Niabot (source) |
 Small section of a large galaxy |
 From Lucky Star |
 Source unknown |
 "Tifa" from Final Fantasy VII: Advent Children |
 Kate Beckinsale as "Selene" in Underworld: Evolution |
 Smarties. Source: PublicDomainPictures.net |
 |
 Fractal I made a while ago |
 Fractal made by someone else |
 Was from a "welcome to imac" background, think series was called iCandy or something |
 Source unknown |
 From Izumo |
 From Pia Carrot |
 From some random background I have |
Additional Resources
Here are some external sites that have a lot of backgrounds
