Regarding DBFZ images
I hope I don't sound rude. I appreciate the effort put into adding transparency to DBS Broly and Bardock images, but unless you can make them as clean as Johhny's works, I don't think it's worth sweating over. It takes way too much work and image clarity is more important than transparency Imo.
If you're still interested in the task. Johhny himself has a guide on how to make these kind of images, and on his sandbox page there are also some DBFZ test images for reference.