The background image for messages on HTC 10 can be easily changed, and the wallpaper will be applied to the thread view when you are texting. To change background image on HTC 10, do as follows:
1. Open the Messages app
2. Hit the 3dot icon on the top right side of the screen and touch to open the Settings.
3. Go to the General settings
4. Scroll down to the Personalize settings, and tap the Thread Wallpaper option
5. Specify whether to select wallpaper from the Gallery or Theme App Wallpaper.
6. Find a suitable image to be set as the background for messages, hit the Apply and you are all set.