You are correct. You will need to use two images.

To replace the background color of an image with a different color:

1. Place the image on Video Track 2.
2. Create a new bitmap image the same size as the first, and fill it entirely with the new color. In this case, you mention wanting a white background, so create a white image.
3. Place your new image on Video Track 1.
4. Add the Chroma Key Color transition to the Transition Track.
5. In the Transition Settings window, choose to replace the black color in your original image.
6. Click OK.
7. Drag the Transition to make it the same duration as the two images.