Good thing then.

Note that this above example is for a Picture object, i.e. indeed the equivalent of single-rectangle billboard polygon objects.

I suspect the general material shaders for other objects might support Alpha too, but I dunno for sure. I know Carrara does.

Also, in this example I think the transparency mask is in its own greyscale image file, i.e. not strictly in an alpha channel of the same file as where the rgb channels are. Same basic idea though: 8-bit transparency mask. You can easily extract the alpha from a 32-bit image with Dogwafle and many other tools including some free ones.