The image in the bot description is the image that will be displayed in the "What can this bot do?" section:
Through the BotFather service
1. Open @BotFather and enter the /mybots command:
2. Select from the list the bot for which you want to change the image in the description:
3. Click the "Edit Bot" button:
4. Click the "Edit Description Picture" button:
5. Submit a new 640 Γ 360 px image or 320 Γ 180 px GIF: