If you are in camera mode, the camera settings only allow you to use the full image sensor which is 4:3.
If you switch to video camera mode, you can then set the resolution to 4K and snap photos and they will all come out as 16:9. The difference is that your snapping a photo of a video clip so you don't get the same options you do as if you were snapping a photo in camera mode. I'm not sure why Yuneec didn't add the two resolutions to the camera mode... pretty much every other camera drone at this price point has it.
thank you both for the comments....yes snerd I think you're correct.
Here a two pictures from this morning. The 4:3 were taken by pressing the camera button while in camera mode... and the 16:9 were taken by pressing the camera button while in 'Video' mode...looks like they were just cropped as you said.