As an administrator of a group you have the ability to personalize your space through the banner and the place image. The banner is the main graphic at the top of your page while the place image displays on the About tab and with place search like below:
To change these graphics you will need to go to your group page and click Manage > Setting
From the settings page you can set the Banner Design and the Place Image.
The Banner Design will allow you to select from on of the preset options, or you have the options of selecting a solid color or uploading your own image. The top banner for groups and spaces is 1440px X 172px.
The Place Image will open Windows Explorer so that you can select an image to upload. The image (group thumbnail) is 200px X 200px or 1:1 ratio; however, when you view the group through the Places tab as thumbnails, that image is 200px X 150px and crops the bottom off if you exceed 150px.