There are 2 ways you can post an image to the forum.
Firstly, you can add it to your post with the 'attachments' system. To do this, you'll see below the writing pane in the full editor a section called 'attachments'. Click on 'Browse', find your image, click on 'attach this file' and your image will be included in the post. Should you then wish to place the image in a certain position in your post, click the cursor to where you want it to go and then click 'add to post' to the right of the screen.
The problem with this is that many images take up much room on the internet (when they say a picture can paint 1,000 words, that also works out about right in kilobytes so 1 picture takes as much room as a thousand words on our server!) so basic accounts don't get much space, full forum members get more.
Secondly, and the recommended method is to use off site hosting. Websites such as http://www.photobucket.com are free to use, very clever and make it really easy to host pictures. Sign up for a free account and follow their upload instructions and once you've uploaded your images you'll see links to the image either as a file eg
which you can post on here using this button above, or you'll see a link to a forum ready image which will be enclosed in [ img ] wrappers eg
which you can paste directly into your post.