Actually, the instructor normally never needs to know what is the ID for his/her course.
If you're familiar with HTML, then when addressing a file in the file manager, you can think of the "<base href=... />" tag as being set to the file manager's root directory withing the course you're in.
In other words, if you have an image uploaded to "lesson_1" folder in the file manager, you could just write:
[b]<img src="lesson_1/image_01.gif" />[/b]
Moreover, putting the full address into the file path is strongly discouraged:
First, you'll get things broken once having moved to other hosting, copied the course, or just restored from a backup.
Second, it is recommended for security reasons that the content directory is moved outside the directories handled by the web server. It has been mentioned several times, e.g. http://help.atutor.ca/admin/new_installation.php?en .