You can change the background image of the Timer by selecting the Profile icon > Settings > Timer Settings > Background Image.
If you wish to upload a photo for the Timer, please ensure you have allowed the app access to your photos through your device settings.
iOS: Launch iOS settings > Privacy > Photos > Locate Insight Timer.
Android: Launch Android Settings > Apps > Locate Insight Timer > Permissions > Set Storage Toggle to 'On'. (This setting may vary depending on Android device).