Sorry to answer your question, but found the undocumented swfrender function (part of swftools) by looking at the sources.
swfrender path/to/my.swf -X<width of output> -Y<height of output> -o<filename of output png>
As you might have guessed, the X option allows you to specify the width (in pixels) of the output, and Y is the same for the height. If you just set one parameter, then the other is selected relative to the original ratio of height and width (quite useful)
This does the trick for me, but since Zarate offered a solution that might be even better (I think of swf to PDF conversion), it deserves credit.
Greetings
das_weezul
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