Image edges are pixelated

One of my colleagues is developing a course using an on screen coach.  The coach is a static image of a character in different poses.  The background on all of the images is tranparent.  Please see below.  Any ideas why this might be happening and what can be done to resolve it?  Thanks in advance.

He writes:

This course animates the image of the instructor in and out to show her in different poses. This works great on all but two screens (see attached Image 1). On these two screens there’s one image (the last one to animate in) that looks pixelated around the edges  when I publish (see attached Image2). I have deleted and re-added this image a number of times with the same result. I even added a shape and filled it with the image. That seemed to correct the issue temporarily, but now it’s doing it again.

I decided that I’d just remove the third “pose” of the character, but when I did that, the second pose started doing the same thing.