Click the "Group Settings" button on the right hand side just above the listed members, click the small image icon on the top left of the banner image or the bottom of the logo image. 

Select a file from your computer, paste a link to an online image, take a picture with your computer's webcam, or upload a picture from Facebook, Instagram, or google drive. Click "Save changes" to apply. 

Keep in mind chosen files must be under 6 MB