Here is a quick rundown as to how to attach a picture you have stored on your computer.
After clicking to reply to a post in a thread -
Once you have clicked on the paper clip button, a window will pop up. That is your manage attachments window. Click on the 'Browse' button.
This will bring up the file upload window. Browse for the picture that you have saved. From your camera or other device. Then click the 'Open' button. In this case I have a pool ball.
This now brings you back to the 'Manage attachments' window. You will see the file path to your image in the top box. Click 'Upload'.
After a few seconds, your image file name will appear below and will be uploaded and ready to insert in a post. Close this window.