Higher tracks take precedence, so a background will typically be on track 1 and the overlay on track 2.
If an image is a .png with transparent background overlaid atop another image, then the other image will be seen in the transparent area.
If an image is high contrast, then chromakey (green screen effect) can key out a background color, which cannot be contained in the image, or it too will vanish.
The dice are a .png file with transparent background on track 2 that was overlaid on a track 1 colored, gradient background.
The gal on track 2, shot in front of a green screen, is wearing no green. Chromakey was utilized to key out the green, replacing it with the track 1 bars.
