How to customise your app icons with Shortcuts app.

Open the Shortcuts app and select Create Shortcut.
Select Add Action.
Type "open app" into the search bar and select it when it appears.
From the new screen, select Choose.
Type in the name of the app that you want to change the image for.
Select the three dots in the top righthand corner.
Type in the name of the app you selected and click Add to Home Screen.
Edit the name of the app (or remove it altogether if you don't want a name listed below the image) and tap the image beside the new name.
Choose a photo previously saved to your phone to replace the old icon.
Select Add in the top righthand corner.
Your new icon should now be clickable from your home screen. You may want to move the original app into a folder.

Image Source: POPSUGAR Photography / Tara Block