How do I trim a picture using Adobe Illustrator CS4?

Hi. I was wondering how can you trim the outline of a picture using Adobe Illustrator. My dad insists that you can using the pen tool in Illustrator but it doesn't work. Can somebody please tell me how?

I'm not sure but I think that you can with the pen tool, once you draw around the area you want/don't want and make it into a full shape by connecting the first and last line together, you can right-click and Make Selection. Once you got that you can press delete, but I'm not sure if this works on Illustrator because I only use Photoshop.

here is an example of what happened after when i used my pen tool on illustrator:

http://i85.www.servimg.com/u/f85/13/75/60/31/untitl10.png

Unfortunately the link isn't working