In your group select Edit. From the right page settings menu click on Avatar and Cover. Upload a new avatar and/or cover and click save. Alternatively. you can change them directly from Group home.

For best results, your Group avatar should be 400 pixels wide and 400 pixels tall, ideally in JPG format, and anywhere between 20-30kb.

For best results, your cover should be no more than 918 pixels wide and 300 pixels tall, ideally in JPG format, and below 50kb.