Commercial Use:
It depends. Some of the permit commercial use, some do not. Generally speaking, if the license type has NC
in it, the answer is no. Even if it does not have NC
there may be other restrictions that effective prevent you from you using it in your situation (for instance derivative uses). You'll need to check the license of each & consider your application.
Attribution:
According to the faq on attribution there is not a set standard:
CC licenses have a flexible attribution requirement, so there is not
necessarily one correct way to provide attribution. The proper method
for giving credit will depend on the medium and means you are using,
and may be implemented in any reasonable manner. Additionally, you may
satisfy the attribution requirement by providing a link to a place
where the attribution information may be found.
While the attribution requirements in the license are the minimum
requirement, we always recommend that you follow the best practices
for the kind of use you are making.
You can read more about the best practices on attribution; here's a summary:
- identify the specific CC license in use
- retain any other legal (copyright, etc) info that was with the orignal work
- double check the details of the legal code for the CC license in use
- make sure your attribution is reasonable and suited to the medium you're working with
Derivatives:
You didn't ask about this, but it's something you need to consider. The odds are likely that you will need to modify some of the photos in some way to fit your card format (that is to say, you're probably not going to find a bunch of photos at the right size & resolution, all using the same color profile, that you can use straight up). Combining and adapting CC material is another area where you'll need to tread carefully:
Whether a modification of licensed material is considered an
adaptation for the purpose of CC licenses depends primarily on the
applicable copyright law. Copyright law reserves to an original
creator the right to create adaptations of the original work. CC
licenses that allow for adaptations to be shared—all except BY-ND and
BY-NC-ND—grant permission to others to create and redistribute
adaptations when doing so would otherwise constitute a violation of
applicable copyright law. Generally, a modification rises to the level
of an adaptation under copyright law when the modified work is based
on the prior work but manifests sufficient new creativity to be
copyrightable, such as a translation of a novel from one language to
another, or the creation of a screenplay based on a novel.
Basically, it hings in part on copyright law which in turn can get complicated, especially if the jurisdiction extends beyond a single country. Make sure the derivatives portion of the license fits your needs.