With the customizing the Safari start page for things like Favorites, Reading List, and Frequently Visited. You can change the background image used by Safari in modern macOS versions. For such, you should read this post about How to Change Safari Background on MacBook Air.
It will allow you to use any picture you want as the default background image in Safari. Also, it offers a neat way to customize the Safari web experience on Mac.
To set a custom background image in Safari, you’ll need Safari 14 or newer on a modern macOS release, like macOS Big Sur, macOS Catalina, macOS Mojave, or newer.