It's not a dumb question. Anyway, all you have to do is:
1. Find the image you want.
2. Highlight and copy the URL of the image.
3. Go back to your profile and type [img]
4. Press CTRL+V or right-click and Paste to insert the URL.
5. End it with [/img]
Here's an example, a guitar:
[img]http://blogs.setonhill.edu/TiffanyGilbert/guitar.jpg[/img]
And that would look like:
Hope this helps.
