Add two html elements to your form. Paste the code below to the first one and place it at the top of your form.

<style>#submitting{
display:none;
}
</style><script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script> <script type="text/javascript">$(document).ready(function() {
$('form').on('submit', function(e){
var submitButton = $(this).find('input[type=submit]');

submitButton.css('display', 'none');
$('#submitting').css('display','block');
});
});</script>

Make an animated gif the size you need and save it to your host.

In the second html element paste the code below. Adjust the src to your image name and location. I placed my html element directly above my submit button.

<img id="submitting" src="http://mydomain.com/images/submitting.gif" alt='' />