Bitmap Gallery - Transparency Button
The Transparency button in the Bitmap Gallery applies the bitmap selected in the gallery as a bitmap transparency to the selection. If no objects are currently selected, the current transparency attribute is set. This means all objects drawn will have the bitmap applied as a bitmap transparency. For details of editing the bitmap transparency, see Changing bitmap transparency.
You can also apply a bitmap as transparency by opening the pop-up menu. over the bitmap (this also selects it) and choosing Apply as Transparency.