PNG image overlay on PBO texture/quad while retaining transparency

Have a PBO rendered on a window created by an external program. PBO rendering using texture/QUAD works fine.

External program displays (overlays) a PNG image on the same window where PBO texture is rendered, but PNG image loses its transparency and shows up with a white background.

Any suggestions on how to get the PNG image to retain it’s transparency when it’s overlaid on the PBO texture/QUAD?