So how to convert this gif animation into a swf file, which the Flash Player could play? (And of course I only want to use open source tools.) It was very easy:

• Make sure you have ImageMagick installed.
• Open a console and go to the directory where the gif file is located:

  convert +adjoin -coalesce loading.gif frame%02d.png

• Now we have a png for every frame of the gif animation. Why png? Because swfmill does not support gif as file format and png it the only available format with support for transparency.
• Create a XML file that look similar to this one:
  1. <?xml version="1.0" encoding="utf-8" ?>
  2. <movie width="32" height="32" framerate="25">
  3.         <background color="#ffffff"/>
  4.         <frame>
  5.                 <library>
  6.                         <clip id="frame00" import="frame00.png"/>
  7.                 </library>
  8.                 <place id="frame00" depth="1"/>
  9.         </frame>
  10.         <frame>
  11.                 <library>
  12.                         <clip id="frame01" import="frame01.png"/>
  13.                 </library>
  14.                 <place id="frame01" depth="1"/>
  15.         </frame>
  16.         <frame>
  17.                 <library>
  18.                         <clip id="frame02" import="frame02.png"/>
  19.                 </library>
  20.                 <place id="frame02" depth="1"/>
  21.         </frame>
  22. ….
  23. </movie>
• Finally merge the single frame images together with swfmill:

  swfmill simple input.xml output.swf

• That’s all.