Reviewing through your inquiry, the nav bar, for example here in the Jive Community:
Is loaded each time a page is viewed, so I would expect your gif to animate each time. However, if you are interested in customizing your instance I would suggest reaching out to the Jive Developers space here in the Jive Community where you can collaborate with other developers on potential customizations you are interested in making to your instance.
Additionally, if you would like assistance developing custom themes or headers for your instance our Professional Services team can work with you to determine the scope of a project and help to develop a solution which meets your needs. If you would like to pursue this option, please submit a case in your customer group here in the Jive Community and the next available support engineer can get this process started.