^ The picture needs to be hosted on a site like Photobucket or uploadit.org. You may also use your own site to host the image. (Do not hotlink!) Once you have hosted it somewhere copy the URL for the image. Then on the BBS click "Reply". Once the "Reply to Thread" screen comes up you need to click on the "Insert Image" icon above the text field. A pop-up window will appear. Paste the URL for the image into the pop-up window's text field and hit "OK". If you pasted in the correct URL, the image will be inserted into the text box. If not, you didn't copy the correct URL for the image so you will need to try copying it again.
For information about image size and rules, please see the BBS FAQ page.