The activity backgrounds will change automatically as they pick up any content that is attached via provocation, for example if your activity has a URL link, the background will be an image pulled from the website in question via Link IQ. This behavior happens automatically.
You may also use the 'upload' feature to attach media to activities, which also clones the attached file to the activity banner. This will not only work for static images, but also animated ones too!
If you have further questions or require assistance with this feature, please click here to create a support ticket.