1. Go to Your Trip Settings
First, go into the edit mode of a trip and click "Settings" at the top.
2. Scroll Through Settings to Find "Partner Logo" Under Advanced
Once the Settings option are open, scroll to the bottom of this window and under "Advanced" you will find the Partner Logo where you can upload an image. Once you add the image, press "Save Changes".
Here is what the final itinerary or proposal will appear as once you add a secondary logo: