Open the Shortcuts app on your iPhone
Tap the plus icon to create a new shortcut
Tap Add Action
Search for and select Open App
Tap App and choose the app you want to customize
Tap the dropdown or info icon at the top
Tap Add to Home Screen
Tap the app icon next to New Shortcut
Choose Take Photo, Choose Photo, or Choose File
Select the custom image you want to use
Tap Add
Return to the Home Screen to see the new custom icon
Repeat the steps for each app you want to customize
Optionally, move the original app icons to the App Library or a folder
Optionally, use a matching wallpaper to complete the theme