(5 years and 1268 days ago)
Source 1 is a render, and not useable.
5.6. 3D Renders or Illustrations as Source:
Using other people's 3D renders or illustrations (even with permission and no matter in which form they are offered) creates a false impression of your artwork. So 3D renders or illustrations can not be used as sources, unless they are entirely created by yourself. Only acceptable non-illustration brushes and clip art may be used, but you need to mention them as a source too. Don't use an image that has been altered by the source owner nor an entire work created by someone else.
Source 2 is from a blog and not offered under a Creative Commons license.
Source 3 is OK but not linked correctly.
Please see: http://www.pxleyes.com/blog/2009/06/how-and-where-to-find-legal-source-images/ and http://www.pxleyes.com/guidelines/photoshop/
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