Transparent Window in PB

We are working with PB 12.5

We would have a window transparent with a DataWindow that contains a Picture that is partially transparent.

We defined a Popup-Window that has BackColor transparent and contains only a DW that fills the whole window. This DW has also a transparent Background Color. It contains a png picture that is partially transparent.

Now  we would expect, that only the non-transparent part of the picture of the dw is shown when opening the window.

Unfortunately there is a lightgray rectangular background, when the window is opend.

Are we making somethin wrong or is it not possible what we want ?

Thanks for any tip