Select the dimensions matching the screen resolution of your monitor (if you don't know, select 1024x768) and follow the instructions below:

OSX Download

1. Select the image size under the graphic.
2. Click/hold on the image.
3. Select "Save Image As..."
4. Save the image to your hard drive.
5. Follow the directions under OSX Installation below.

OSX Installation

1. In System Preferences, select "Desktop."
2. Under the Collection menu, select "Choose Folder..."
3. Select the downloaded image from your hard drive.
4. Click "Choose," and close the Desktop window.