to determine your current screen size:

1. [mac instructions here]

to install new wallpaper (OS 8.0 or higher):

1. click on one of the screen sizes above to open the full-size image
2. click and hold on the full-size image, then select "Save this Image as..."
3. save the file on your hard drive
4. under the Apple menu, select Control Panels, then select Desktop Pictures
5. go to "Select Picture" and open the file from the location where you saved it
6. select "Set Desktop" and the image should appear on your desktop