When you make a new post, scroll down the screen past "Submit New Thread" and "Preview" and you'll see a field called, "Manage Attachments". Click on that and it will prompt you to browse your drive for pictures you want to upload.
If you want to post them in an existing thread, click "Go Advanced" and you'll see that "Manage Attachments" option appear below the "Submit/Preview" option and you can do the same thing.