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glpuzzle Source-code:

Licensed under GPL v2, see GPL.txt

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Images:

 - train, blackberry, bike, goat, gorilla, horse, bird, clouds, castle: 
   
   Photographs by Maarten de Boer
   Licensed under Creative Commons Attribution 1.0 License
   http://creativecommons.org/licenses/by/1.0/

 - heli:

   Photograph by Joana Clotet
   Licensed under Creative Commons Attribution 1.0 License
   http://creativecommons.org/licenses/by/1.0/

 - penguin:
   Obtained from:
   http://commons.wikimedia.org/wiki/Image:Penguins_111602.jpg
   Licensed under Creative Commons Attribution 1.0 License
 
 - penguins:
   Obtained from:
   http://en.wikipedia.org/wiki/Image:Penguins_Edinburgh_Zoo_2004_SMC.jpg
   Licensed under DGPL

Creative Commons Attribution 1.0 License: 
http://creativecommons.org/licenses/by/1.0/

DGPL:
http://en.wikipedia.org/wiki/Wikipedia:Text_of_the_GNU_Free_Documentation_License