No clue what's wrong there
I don't use photoshop so can't really help you there.
One thing, I always start with a fully transparent background. Some softwares have issues with layers so merging or flattening can help sometimes. You can try inverting the alpha channel. I can't say without actually seeing it and testing.
Here is a simple plant you can check out to see if it may help you get it figured out.
Edit: missed the pic of the plant. If you are trying to make that image into a plant, select the black part and paste it into an alpha channel. Then expand or contract, don't remember, to make the area you "DO" want to see pixel or two smaller. Then invert the selection so you have the area you "Do" want to see selected. Then dump white into that selection. save it out as your dds. Then open it right back up to make sure that alpha channel is "Still" there. gimp sometimes forgets all about that channel on export. I hope you get it figured out. OR someone with way better explanation skills helps out