Did you try editing the image to make sure that the background is really transparent? Were there any messages at the bottom left of the screen?
It is because the edges aren’t following the pixels in the image.
You can do that without employing Image Trimmer at all. Import the image and make it quite large. I like to temporarily edit the style to make Profiles very thick and give them a color that will show easily over the black of the image. Then trace around the image with the various drawing tools. When you close a loop of edges you’ll get a face. I haven’t closed the loop here.
