To be able to set a custom background image in Safari, you’ll need Safari 14 or newer on a modern macOS release, like macOS Mojave or newer.
• Launch Safari.
• Now, simply right-click (or Ctrl+Click) in the empty area on the start page and click on Choose Background. This will open up a file selection window.
• Find the picture that you want to set as the background from the file selection window and select it. Now, click on Choose.
• From now on your start page will look completely different.
If you feel like removing the custom background at any point, just right-click (or Ctrl+Click) in the empty area and choose Clear Background.