Get into canva, and select the facebook cover option. This will give you a cover which is sized to desktop: facebook covers are not responsive so keep that in mind when you design, as your edges will disappear.
Upload and drop your background image onto the canvas by clicking on it once. You can resize by clicking and dragging the corner. To maintain aspect ratio, hold down ctrl (otherwise it’ll get squished out of shape).
Click on the image you want on top and it will appear on the canvas. Notice how if you move your image too far off canvas you can lose layers? Ctrl + Z is your friend: just keep playing until you get it right. Adding an image will always result in the new image on top, so use the arrange button in the top left to send it to the back again if need be.
When you’re done, save and download as .png.
If you find your graphic is grainy when uploaded to facebook, you will need to remake it at double the canvas size. Go to the tutorial to make you about image to find out how to make an image based on a custom canvas size or how to correct a blurry image.