1. copy your image (Ctrl+A and Ctrl+C)
2. make a new document-sized pure-black layer behind it
3. group the black layer and yor image together
4. add mask to the group
5. enter mask edit mode (alt+click on the mask icon/thumbnail)
6. paste your image in the mask (b/w) and then invert it.
7. save it as a 24-bit transparent PNG