Customizing the various app icons on your iPhone is a great way to personalize your phone. Doing this is a fairly simple process by way of Shortcuts.
Open the Shortcuts app.
Tap on the Blue + icon at the top right corner of the screen.
Tap Add Action.
Select the Scripting icon.
Select Open App, and then Choose the app icon you'd like to customize.
Click Next at the top-right of the screen.
Name the Shortcut/icon and click Done.
In the Shortcuts window, tap on the 3 dots (ellipsis), for the new Shortcut, then click the 3 dots again and select Add to Home Screen, then click and hold on the icon under Home Screen Name and Icon and select Choose Photo.
After choosing the photo/icon you'd like to use, click Add, then finally click Done.