The provided png example is 24 bit so does not have any transparency.
That you perhaps experience it as transparency is another ballgame. I do not have enough knowledge on bgrabitmap to be able to have a quick answer for you. As a guess you want to change the black background (r, g and b values are 0) into another colour. Which colour you want that to be i have no idea. So check for r,g and b being zero and change only those pixels who have all r,b and b values set to zero at the same time, into another colour of your choice.
In short a true transparent image has a additional channel (called alpha) which tells how transparent a single pixel is. There can be 3 states: not transparent at all, completely transparent, or somewhere in the middle.
Think of it as a see through glass window. When all pixels are completely transparent then you can see through the window. If you would be able to draw a pixel on that glass window with a black paint then this pixel would not be transparent at all. Would you for instance draw a pixel with red oil-paint then this pixel could be seen through, although the color is still reddish. The magnitude of which the pixel can be seen through depends on the value of the alpha channel.
Again, your picture has 24 bits colours, so has no transparency information added. Therefor it would need to have 32-bit colours.