If you look at the actual license for BY-NC 3.0 here:
http://creativecommons.org/licen...It says:
You may not exercise any of the rights granted to You
in Section 3 above in any manner that is primarily
intended for or directed toward commercial advantage or
private monetary compensation. The exchange of the Work
for other copyrighted works by means of digital
file-sharing or otherwise shall not be considered to be
intended for or directed toward commercial advantage or
private monetary compensation, provided there is no
payment of any monetary compensation in connection with
the exchange of copyrighted works.We realize that people have different views on what non-commercial SHOULD mean. We will review this clause again when we upgrade the agreements. However, it should be noted that while 2/3 of the licenses people choose have the NC restriction, I don't personally know of any significant issue resulting from the NC ambiguity. Basically, as with HTML, when you write it be strict, when you interpret it, be liberal. In other words, if someone is using an NC license, it means that they are sensitive to commercial exploitation. When in doubt, ask. Conversely, if you're really worried about commercial exploitation or someone using your work in a way that they might not think is commercial, but that you would, don't use a CC license.
If someone wants to rip off your work, they will rip it off whether you have a CC license or not. If someone is going to care what you think, they'll look at the license and the NC choice will signal that you have a sensitivity that they should be aware of. Just remember that technically, if you read the license above, you could interpret it to mean that your image could be used on a blog with ads or shared on a commercial site such as YouTube if those ads are supporting the site/infrastructure. For instance, putting money into a copy machine doesn't make the photocopy of some page a "commercial use" transaction but the money is just supporting the infrastructure.