There are two ways:
1. Use the Manage Attachments option just below the Submit Reply button if you want to use a picture from your hard drive. This,however,will not automatically show the picture in your post,for that you have to use the second option:
2. Go to an image hosting site and give your picture an URL,copy it,and paste it to the Insert Image window(the small yellow-ish icon,you'll find it easily.)
Hope this helps.

