To make your own SummerBoard theme follow these steps:

1.  Create a new folder with the name of your theme e.g <Example>

2. You need to create 3 images for your theme:

• StatusBar (optional, 320×20 PNG, alpha)
• Wallpaper (320×480 PNG, no alpha)
• Dock (320×91 PNG, alpha)

(alpha = Transparency)

These need to named in the following format:

• Music.png
• Photos.png
• Videos.png

etc.

TIP*

If your using Photoshop use the “Save for Web” option (Shortcut keys: CTRL + SHIFT + ALT + S)  This will reduce and optimize your icons for your ipod touch.

Source: http://www.apptapp.com/