Click Edit, then Change This Post, making sure that "I want to preview my modifications and add or edit an attachment" is checked.

Now under "You may attach a file to this post while editing" click "Browse" and find the picture, and double click on it or click on it and click "open". Then click "Continue", then "View Post". At the top of the post, click on "Attachment". Now copy the URL at the top of the browser bar. It'll say http://www.4x4wire.com/forums/post-images/your-image-here.jpg or whatever. Highlight it and copy it. Now go back to your post, edit it again, and under "Instant UBB Code" click "Image" and then paste in the URL you copied a moment ago and click OK. Then unchedk the "I want to preview" box and click "Change post".

Its a lot of steps but it goes quickly once you know how.