Are reviewers chosen from among the editorial board of a journal? If, in a certain international journal where I have submitted, the journal webpage lists the editorial board, should I expect that the possible reviewers of my paper are selected from among the members of this board? 

Or is it possible that the Editor in Chief may choose an "outsider" to review my submission? 

I didn't indicate any preferred reviewers in submission.
Generally, when a paper is submitted to a journal it is first assigned an editor. The editor then identifies a qualified outside reviewer for that particular manuscript, and invites him/her to review the paper.

So the answer is: No, you should not expect that the possible reviewers of a paper are selected from among the members of the editorial board.