1) Determine Your Resolution

For Windows Users: Checking your resolution is easy. Double-click on your My Computer icon and select the Control Panel. There should be an icon labelled Display.
Select this, then click on the Settings tab at the top of the window.
Just over half way down this tab are two controls - "Color palette" and "Desktop area". The numbers presented under the slider in the Desktop area are your resolution. Common resolutions are 640x480, 800x600 and 1024x768.

For Mac Users: From the apple menu select "Control Panel", select "Monitors", see "Resolution" for desktop size. (eg. 1280 x 1024, 85Hz)

2) Save the file of your choice to your hard drive (choose the version that matches your resolution).

This can be done by either clicking on the a file, letting it load in your browser and then right clicking on it and choosing save as, or right clicking and choosing "save target as" or something similar to that.  You may skip step 3, if you are a windows user and when right clicking can select "Set As Wallpaper".  Make note of where on your hard drive your save the file.

3) Install

For Windows users: Right click your desktop and select Properties select the Desktop tab and then select the wallpaper from the menu.

For Mac Users:  Open Control Panel "Desktop Pictures".  Click remove picture and then click select picture.  Load new wallpaper from hard drive  then "position automatically" for wallpaper.