Clipboard Transparency


I have a problem with the System Clipboard.
I have a BufferedImage with type


I use the following for copying the BufferedImage to the systems clipboard:

TransferableImage trans = new TransferableImage(sc);
Clipboard c = Toolkit.getDefaultToolkit().getSystemClipboard();
c.setContents(trans, this);

with TransferableImage implementing Transferable.
When I try to paste the image in a program like PowerPoint for example,
all that should be transparent is black.
But when I save the image by

ImageIO.write(image, "png", file);

it still has its transparent parts.
So my question is:
Where is the transparency lost and how can I prevent that from happening ?