A cover is optional, but a relevant image can make your page easier to recognise. Show the product screen, the artwork, the event space or a detail of the thing you made. Choose an image you have the right to use. A generic stock scene may look polished, but it should not imply a result or a place the destination does not show.
Check how the image crops inside the page card, especially on a phone. Important details should remain visible at a small size. Avoid placing your whole message inside the picture: the text may become unreadable, and the title and message already have that job. If the image is mostly decoration, consider using the page colour instead.
Uploaded covers are reviewed with the page before they become public. A custom creator photo is also reviewed with that publication; you can use a site-provided avatar preset without uploading one. Keep both images safe, truthful and relevant so that the visual introduction matches the page people actually open.