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:
Which is so very obviously based upon this Gil Elvgren painting,
...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:
Whaddya think?