Art And Rights

US Copyright Law states that unless the artist explicitly signs over copyrights, they retain them even if paid to create the image. Most furry artists don't sign over the rights, so by default they are well within their rights to ask you to take down an image, even if you paid them to make it for you. Many people point to some "work for hire" clause and say that paying them means they hired them, but this is incorrect unless you've specifically made them an employee and are paying them a daily wage, with all the business ownership tax fun that goes with it. The alternative is contracted work, in which case one would need to ensure the contract includes verbage that specifically assigns copyright of the work to the person.

Actually, Copyright law distinguishes between "work for hire" and "employee". They don't have to be an "employee" (hourly wage, tax setup, etc) in order to be contracted as "Work for hire". Reference Title 17 Section 101 for the definition of "work for hire", and Section 201(b) in Circular 92 at copyright.gov.
http://www.copyright.gov/title17/circ92.pdf

There you will find that a "work for hire" means quite a lot of things that have nothing to do with being an employee in the manner you describe, but relevantly:
"a work specially ordered or commissioned for use as a contribution to a collective work" (the commissioner's gallery is not a collective work, btw), and:
"if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire."

There have been actual cases where email is considered sufficient written instrument. This is actually how I acquired the copyright to my banner image, though I have not yet tested the strength of this by registering it formally. When I do so, I have to provide the name and address of the original artist, presumably so the Copyright Office can contact them and say, "You good with this?" ;)

(Note: Copyright law is actually kind of vague on what constitutes an 'employee', though they give examples of elements to look for, like what you stated; but they're not enough on their own .. otherwise every artist who had hourly rates would be an employee of the commissioner, which isn't so).

Relevant to this discussion, the Copyright Office was soliciting input from the public as to how to handle "Visual works":
http://www.copyright.gov/policy/visualworks

The deadline passed (October 2015), but this suggests that they're reaching out and desire to clarify how to handle such things as graphics illustrations, which this site is all about! I guess we can hope to see some Copyright Law updates as regards art commissions.

Lusetifan -- unless you bought the copyright (usually expensive!) or had a mutual agreement of transfer of copyright (see "work for hire" discussion above), the artist owns the copyright to the image at creation time, and continues to own it by default. What they let you do with it is up to them. This includes re-posting it (you should always ask permission to do that, and that's not just a courtesy but a legal requirement), and printing it, and publishing it and so forth, even if it's not for any profit or revenue.

Technicalities and legalese aside, it's simply good courtesy to honor the creator's wishes. You bought the commission because (at the time) you respected them, their work, their quality, and/or their name. You should also respect their wishes. It will earn you a greater rapport among other artists than you would if you put up a fight.