Hi.
If your animation is in the main timeline, go to File > Export > Export Movie... > PNG sequence. But make sure your timeline is extended. Make sure it has more than one frame. Because in some cases the animation can be contained inside of a Movie Clip, but when there is an animation inside of a Movie Clip, only the first frame will be exported.
You can also right click a single Movie Clip and choose to export a PNG sequence from its frames, if its timeline has more than one frame.
Alternatively, you can go to File > Export > Export Video..., check "Convert in Adobe Media Encoder" and then, when the video opens up in Adobe Media Encoder, create a PNG sequence from there. This method has the advantage that exported videos record everything that happens on the stage, even if it was caused by ActionScript.
Regards,
JC