Mark, it's easy... in brief:

Uploading a Picture to the BB

1) Find a picture on the internet.
2) Right-click on it, and Save to your desktop (so it's easy to find)
3) Go to http://www.tinypic.com/ and hit Choose File (choose the one you just saved to your desktop)
4) You should then be presented with links, choose the "Forum" link
5) Copy/Paste that link into your post.

You can test the process out first in the The Official "Test" Thread in the Help Forum to be sure you did it correctly