Agencies want you to hold copyright to the whole image.  'commercial use' means like an ad, not in a piece you will sell for stock.

You don't own the copyright to any CC work, and thus you couldn't say "yes" to the term in most supply agreements that asks that you own the copyright to every component of the submitted image.

See some examples here:

http://submit.shutterstock.com/guidelines.mhtml
http://www.dreamstime.com/terms
http://www.123rf.com/submit/agreement.php

From Shutterstock's terms:

"Submissions must be wholly owned by the submitter. Found or public domain images or footage cannot be submitted under any circumstances. If you do not have complete rights to the submission, you may not submit it.

Submissions must NOT contain any copyrighted material including paintings, other copyrighted photos, copyrighted logos, or any other art/advertisements/sculptures/exhibits or audio which are copyrighted. If submitted material contains any of these or other types of copyrighted content, you must either submit releases from the copyright owners or you must mark the images as editorial"

Thanks guys! Great answers as always!!

But what about the images in the NASA public domain library?

So if i interpret that correctly, PD images is ok if you can show that it is just a small part of your work and that you can show a link to the PD images proving that they are indeed PD...!?

Public Domain images usually require a credit no different than creative commons images....at least the public domain images that I have seen released in the U.S. that were created by U.S. Government entities (including NASA - which is why Shutterstock has the requirement pointed out above).