From this thread go down to the quick reply box at the bottom, click Go Advanced, that will bring you to a new page. A bit below the type your message box you should see something that says Manage Attachments. Click it, that will bring up a new window with browse buttons.
Click one of the buttons, locate the picture you want to upload on your computer, and then double click it. Then hit upload on that box, once it finishes uploading you should see the file listed on the original window under the attachments section, then click submit.