To change the chat wallpaper for all chats, go to Settings → Chats → Chat wallpaper. From there, you can choose one of the available Zangi wallpapers or tap Choose from Photos to use an image from your own device.
You can also change the wallpaper for a specific chat only. To do this, open the chat with the contact you want, tap on the name(IOS) or on the three dots on the upper right corner(Android), and choose a custom wallpaper for that conversation.
To personalize and change the chat wallpaper for specific chats: go to the contact’s or group’s profile info → Chat wallpaper.