After you've loaded your bmp in the Material-Editor, you have to search for a little Triangle above the Color settings.If your Mouse is on it, it says Materialoptions in the upper right part of the Material-Editor.Click on it.In the second Row of the Menue it says something like cutout Transparency- click on it. After that, click on the transparency row in the Optic section and choose texture A. It should work now.