Hi Nancy,
Yes it is definitely allowed! Stock illustrations are a huge market and the good stuff generally sells like hotcakes.
To submit illustrations, log into your Artist account and go to
http://www.zymmetrical.com/Account/Submissions/Graphics/tabid/119/Default.aspxThe 'cleaner' your illustrations are, the more likely they will be marketable. For example, if you have a flat illustration originally done by hand, it might be worth the extra effort to trace it out and make a vector version.