Copyright holders may be the author, the institution, a publisher, a distributor or other entity. If you sign over the copyright for your article to the publisher, then the copyright holder is the publisher, and the publisher decides what it will allow to happen with your article. If you maintain your copyright to an article or any portion of the article, e.g. images, then you control what happens with the materials for which you have retained the copyright.
The copyright holder has the sole right to do any of the following five acts:
Copy, duplicate, transcribe
Sell, lease, lend, rent -- including distributing in the classroom or allowing downloading
Make modifications to create a new work
Show audiovisuals, video or sound clips (this includes showing audiovisuals and playing of audio clips)
Make a picture, photograph, or graphic art available for public viewing