First, save an image to your desktop by right-clicking on a thumbnail below and choosing Save Image as...

Then, to add or change the cover photo of your Facebook timeline:

1. Go to your timeline
2. Hover your mouse over the cover area of your timeline then click Add a Cover or Change Cover
3. Choose whether you want to upload a new photo or pick a photo from one of your existing photo albums
4. Once you choose a photo, you can reposition it by clicking on the image and dragging it up or down
5. Click Save