First of all you need to see what setting your desktop is set at. To do that you need to right click on your desktop and pick "properties", when the menu appears click the last tab on the right called "settings", and then you can see what the setting is for your desktop. i.e. 800 x 600. This is the most popular setting, but please check and pick the one that is right for your computer because you will be able to see all of the image and if there are words they will show up correctly.
Then click the desktop size under the picture you want and then go on to the next step.
For Your Windows Computer
1. Right-click on the image of the wallpaper and choose "Set As Wallpaper" or "Set as Background"
For Your Mac Computer
1. Save the image of the wallpaper to your Mac.
2. Open the Appearance control panel.
3. Select the Desktop tab.
4. Drag the image to the top of the control panel.
5. Click the Set Desktop button.