Your Dockwa Marina Profile is directly connected to the Marinas.com profile associated with your account. In order to change the photos on both profiles, you must first claim your marina on Marinas.com. You can find instructions to do so here:
How to Claim your Marinas.com Account
Once you have access to your marina's settings on Marinas.com, you can easily add and delete photos from your account under the "Photos" tab on the left (See Below).This will directly alter your Dockwa Profile as well.