When you start responding in the quick reply box, look below it and click "Go Advanced". At the bottom of the page it takes you to, you can click "Manage Attachments". This takes you to a popup window where you can browse the files on your computer to select one at a time, and then click "upload". You can then browse and upload up to 5 pictures within that same reply. Go back to your message on the main reply page, and "Submit Reply". Your pictures should appear. Just make sure they're within the 500x500 size limit.