Click on the link below that matches your monitor’s resolution. Right-click and choose “Set As Desktop Background…”.
Click on the link below the image you want to use. Hold your finger over the image. Choose “Save Image”. Go to your Camera Roll. Choose the image. Hit the arrow icon in the bottom left corner. Choose “Use as Wallpaper”. Move/Scale if needed, then hit Set. Then choose if you want to set just the lock screen, just the home screen, or both.