To change a group cover image in Experium Business, click on Groups in the left menu and then click on the appropriate group. Then click on Settings (1).

Click inside the image area and then select the image you want to upload. Click either in the search online / filter by name area to upload a stock image or click on Choose a file to upload your own image. You can then position the picture to suit your needs. You can do the same for the group logo by clicking on it (3) and changing.