Well, I don't have access to PS at the moment, but after you convert to greyscale, you'll probably want to up the contrast a lot so that the flowers are pretty much blown out, and the background is near black. That should give you a pretty decent bump map. If parts of the flowers are getting darker instead of lighter, maybe select all the flowers (using magic wand or the like) and increase the lightness instead of just the contrast. Hope that helps.