Type your message, then before SUBMIT NEW THREAD button is selected, scroll down a little more, until you see MANAGE ATTACHEMENTS.
Click that, it will bring up another screen. At the top it says to Browse your computer to Upload the files.
Once you have found your file, click the UPLOAD button. It will now appear in a list under CURRENT ATTACHMENTS.
You can add only 3 at a time. Once you have the 3 you want, click CLOSE THIS WINDOW.
You may then SUBMIT NEW THREAD.
Once you open the thread, you can then edit to post more pics. Select the GO ADVANCED option.