You start by clicking reply to a post or start a new topic.
So Click on 'Upload attachment' toward the bottom of the page:-
Next click the 'Choose' button.
This will open a folder view, you then need to locate the image on your hard drive usually in 'My Documents', then select the image and click 'Open'.
Then click on 'Add to file'
The page will reload, now you will have a new section below your message window 'Posted attachments'.
Next go to where you would like to add the image in your message, and click 'Place inline'.
Then your image is show like this in your edited post.
Then carry on editing your post, or click submit.
This needs tidying.