I was talking about something like this:
http://www.ghostinthesheet.com/dialog.jpgmetamorphium, how do I create that transparent rectangle that is placed behind the actor's portrait and contains the dialogue text?
What I do in GIMP is create an image with the right dimensions, filling it with black colour and reducing the layer opacity. In GIMP it shows it as I wish but exporting it to .png creates a grey image. I do not understand what you mean by RGBA.
As for why it was not visible when I added the image in the Window, I think that particular .png I tested was exported in a way that Wintermute cannot recognize.
Thank you all for your responses and I hope you can help me further.