You cannot do it in Quick Reply...
You have to use the reply button (at bottom of thread above Quick Reply). Click reply which will take you to Post Reply. Below the message section is Additional Options. Click that on and you will see the Attachment Option. Note the pic can only be 128KB and not more than 4 pics. Browse your computer for the picture and attach it. Do that last after you've done your message etc. It will not allow you to preview without having to re-attach the pic.