Open the Messages app
Open the conversation you want to customize
Tap the contact name or group name at the top
Tap Info if shown
Look for Background, Wallpaper, or Customize options
Select a background or wallpaper if available
Save or apply the change
If you do not see a background option, iPhone Messages does not support changing chat backgrounds in the built-in app
Use a third-party app only if you want custom chat backgrounds in a different messaging app