US Copyright Office Circular 14: Derivative Works notes that:
To be copyrightable, a derivative work must be different enough from the original to be regarded as a "new work" or must contain a substantial amount of new material. Making minor changes or additions of little substance to a preexisting work will not qualify the work as a new version for copyright purposes. [...] Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work. The owner is generally the author or someone who has obtained rights from the author.
So, when I saw this painting:
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Which is so very obviously based upon this Gil Elvgren painting,
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...it got me to thinking. It's my own opinion that the new painting is not different enough from the original to qualify as a copyrightable derivative work, although Dave Steele, the artist, does very clearly state on the page that I pulled it from (Fair Use as a citation) that it is
his copyright. Even if it is considered different enough to qualify, did he actually get the permission from the Elvgren estate to create a derivative work? Would not getting permission nullify his copyright claim?
For illustrative emphasis, I've overlaid the new painting on Elvgren's:
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Whaddya think?