This is not something you can do in the Admin console, Brandkit Support will need to configure this for you.
To change the graphic used as your watermark, you will need to contact us here at support or your Brandkit Account Manager to arrange.
You will need ti supply the graphic you wish to be used on a 780 x780px transparent canvas in .png format. Alternatively if you supply us a Photoshop and vector file we can create the Watermark .png for you.
Example Watermark for Tourism New Zealand below.