Assuming you have the pics on your computer, simply use the "Go Advanced" button to the right of "Post Quick Reply" just below the message you're going to post. When you get to the "Go Advanced" page, find the paper clip in the center of the top line of options. Press on the paper clip and it will take you to the "Manage Attachments" window at the top left.
Then, click on one of the "choose file" buttons and you can browse your PC for the pics you want. Once you find the file on your PC, click on it and choose "open." That file will then show on your "Manage Attachment" window. You can upload up to 9 pics. Finally, click "Upload" in the lower right of the window and you're done by clicking on "Submit Reply." The pics are a larger thumbnail which you can enlarge multiple times by clicking on the pic.