Can't get any of the trim tools knife scissors or pen tool to trim a png file

I imported a PNG image into illustrator to do some trimming.  The image is of a fish from which I want to trim the dorsal fin and the tail and just set them aside until after I've done some work on the body.  Then I will reattach the objects and I'm done.  I used the pen tool to remove the dorsal find and the tail then selected the fish and used the divide path tool to separate the afore mentioned items but nothing happens.  Please tell me what I'm doing wrong cause I'm not a pro but I love the things I can do with Illustrator.  I await your response.