Regarding Creative Commons (the most usual license of the kind for graphic assets), you can add any icons or graphics that allow commercial use.
For example, a Creative Commons license that is NOT NonCommercial is allowed for paid software. You generally (almost always) need to attribute the author, but using CC graphics doesn't necessarily mean you need to distribute your product with the same licence.
The restrictions will depend on the type of license the creator chose to distribute the art under, each piece will have a different one. You can check all the CC licenses here: http://creativecommons.org/licenses/
A Creative Commons licensor answers a few simple questions on the path
to choosing a license — first, do I want to allow commercial use or
not, and then second, do I want to allow derivative works or not? If a
licensor decides to allow derivative works, she may also choose to
require that anyone who uses the work — we call them licensees — to
make that new work available under the same license terms. We call
this idea “ShareAlike” and it is one of the mechanisms that (if
chosen) helps the digital commons grow over time. ShareAlike is
inspired by the GNU General Public License, used by many free and open
source software projects.
