Hi,
I'm not sure what the story is with billboard objects. You're right that Walli is using billboard TGO models in his tutorial, but I'm not sure where they've come from. You won't be able to import the TIFF billboard images into a populator, as you've found.
In any case, it is important to point out that TG2 - Xfrog plants bundles are bundles with products from Planetside and Xfrog. Xfrog is responsible for all the plant related content, so questions related to the plant content are best addressed to them. That's not so say we won't help if we can, of course.
Regards,
Jo