To find out how large your screen is...
Click with the right mouse button on your desktop and a pop-up menu should appear. Select 'Properties'. A box will appear with tabs across the top. Select the 'Settings' tab and on the right-hand side of the box, it should say 'Desktop Area' and this will tell you your screen resolution.
To download the image...
From the webpage, click on the link to your left that is the same size as your screen, save this image to your desktop.The picture should now load in your browser. Then right click on your desktop, go to the properties menu. Select the 'Desktop' tab and browse to choose the downloaded image. Stretch as required and select the "OK" button. Your desktop image now should be updated.
OR Open the link to the same size as your screen, wait for the picture to load in your browser and right click on the image, "set as background" in the menu.