Setting your newly downloaded wallpaper as your desktop background
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Please note I am unable to give you instructions for Mac users, as I have 
never used a mac in my life!

In Windows:
1) Choose the image which is the right size for your computer. The number
shown in each file name is the width of the appropriate resolution. (For
example 1024 means it fits the resolution size of 1024 × 768.)

2) Extract the chosen image to an appropriate folder (preferably one you
can find).

3) Right click the image in Windows Explorer and select "Set as Desktop
Background".

To find out your screen resolution in Windows:
1) Right-click on your desktop and select "Properties"

2) Go to the "Settings" tab in the window that appears.

3) In the "Screen resolution" box, your current resolution is displayed.