For example: http://commons.wikimedia.org/wiki/File:Epithalamus.png
Could I take an image file like that, splice it apart, and use it in an application that would be sold?
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The only problem with “Public Domain” (as it relates to any kind of resource) is that you don’t get an explicit license for using it. Rather than giving you explicit permission by the author or copyright holder, public domain simply states “You don’t need permission.” Anyone can claim that a resource is in the public domain, even if they are not the original author.
For some organizations, that’s not quite good enough. That’s why many software developers prefer providing an explicit license, even if it’s the "Do whatever the f_ck you want to with it" license.