Open the Photos app and save the image you want to use
Open the Shortcuts app
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 share icon or settings icon at the top
Tap Add to Home Screen
Tap the icon image next to the app name
Choose Choose Photo
Select the image from your Photos library
Tap Add
Repeat for each app icon you want to change