You can get to the Trip Settings by clicking either on the name of the trip, or on the Settings button at the top of the screen.

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: 

The recommended dimensions for the partner logo should fall within the range of 375x80 pixels to 670x80 pixels. Additionally, we recommend using horizontal logos for this feature, similar to the one displayed previously.