Select "Go Advanced" from the blue box options.
Scroll down to "Manage Attachments" and click on that box
In the new window, click on "Browse"
Find the picture you want to attach and double-click on it.
Now click on "Upload"
When the picture is uploaded its name will be shown in the "current attachments" section. (If there is a problem you will get an error message that the file could not be uploaded.)
Now you simply select the "Post Reply" button and wait.
DO NOT SELECT PREVIEW!