- 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.
Get the full step-by-step guide to customizing apps (with pics) here.