It still didn’t drop for me even in rgb mode.
What worked for me is if you
1) Add whatever emitter you want to the canvas.
2) Right click on that layer (after making sure to check Windows>Timeline )
3) “Add Image Mask”
4) At that point as long as under the Emitter tab Shape>Image is set.
5) A big drop zone is added to your canvas for you do then drop the image you want to use there, rather than under the Inspector tab.
That should work for customizing all the built in Motion Effects with your own images.
(oh and yes PNG files worked for me). Didn’t try other stuff.)