http://pixabay.com/it/service/terms/#download_terms

Downloading and use of Images
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CC0 Via download provided Images on Pixabay are bound to Creative Commons Deed CC0. To the extent possible under law, 
uploaders of Pixabay have waived all copyright and related or neighboring rights to these Images. You are free to adapt 
and use the Images for commercial purposes without attributing the original author or source. Although absolutely not 
required, a link back to Pixabay would be nice. 
