If you are the Admin of your group (either BASIC or PRO), you can add a background image at the top of your group homepage. Simply click the edit icon at the top right of your page, and follow the prompts. Your background image should fit the dimensions of the space, and not be too big. In general, the following dimensions and size are good guidelines:
Dimensions: 1200 x 300
Size: < 100KB
Using your company logo: You can customize your group homepage by adding a backdrop image to the header. This is a large, widescreen image. If you'd like to use a logo, you must create a wide image that has that logo embedded in it so it is not stretched to fit the space.
Here's an example of photographic background. You can see that the system will automatically display your group name in large, white block letters.
The next example shows what's possible with a simple graphic instead of an image. This group was clever in that they used a white background, so the system-generated name is not visible, and, instead, the user's focus is on their logo and custom lettering just above.