kinan:
Your image needs an
alpha channel for the transparency. There is some information about this in the help files: for more information with images, go to Help > ArchiCAD Help, when your browser opens click on the "Search" tab, and input "alpha channel" in the search box, in the results list "Material Texture Panel" will lead to detailed information, at the bottom of this page open the
Alpha Channel Effects section. You can also search this forum for
alpha channel.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David