Poser's internal Flash is not worth using.  It's extremely slow and inflexible,
and requires you to set color limitations that aren't really needed.

Much better to render to separate frames, then use an external app to 
combine them into SWF.

My favorite is the little free utility MotionSWF, available here:

http://www.vector.co.jp/soft/win95/art/se226046.html

Here's a short instruction text for MotionSWF, since the 
original  instructions are in Japanese:

http://www.flashkit.com/tutorials/validate/3rd_Party/Using_Mo-Johnie-593/index.php

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