Tim Hardwick, MacRumors, on macOS Safari 14:

One of the most dramatic changes is the overhauled Start Page, which now includes a browsing privacy report, iCloud tabs, Siri suggestions, and more. It also features several customizable options, such as the ability to choose your own Start Page wallpaper.

If you fire up a new, blank page in Safari 14 (your “Start Page”), you’ll see an icon in the bottom-right corner of the page. Here’s an image so you know what to look for.

Click the icon, a pop-up will appear, filled with checkboxes to enable/disable various Start Page elements. You can also choose a Background Image from those presented, or click the + and select any image you like.

Except “Catalina Nights.heic”. Don’t choose that one. You’ve been warned.