I don't know what to say, I just tried an image sequence (3 frames from a 12 frame anim) and it was antialiased. In the render settings I checked antialias, in the movie output I selected image sequence and Poser 4 as the renderer, the only other thing I did (and I don't know if it makes a difference or not) was select Poser 4 from on the dropdown on the render tab in the pose room.
What format are you saving the images in? If it's .jpg that could explain the lower quality as Poser has a pretty poor .jpg export.