The backbuffer does have alpha. I'm not sure if it has alpha using Graphics but it does if you specify BGL_ALPHABUFFER as one of the flags for bglCreateContext. It's perfectly possible to grab any of the buffer content RGBA or even just A, into a pixmap. Just make sure the pixmap you grab into is, itself, already set up to have that format. If you loaded a plain RGB image it wont have an alpha channel or space to store alpha. You should look at ConvertPixmap() to make sure it become PF_RGBA8888 before you do the grab.