If you are new to computers or are just not sure how to select images as wallpaper, this guide should make things clearer for you.

Extra bits
Different operating systems might work slightly different, but ultimately the process is the same.
Windows users can also open their downloaded image in paint and use the 'Set as background' feature. For best results, select stretch to fit. This will automatically fill a larger monitor or resize an image that loses its edges on a smaller monitor so that it displays correctly.