Upload them to photobucket.com or imgur.com. Then copy the URL of the photo by right-clicking the photo and choosing "copy image url" or similar based on browser. The upload site may also provide this in a more obvious way when viewing the photo.
Then click the insert image button when creating the thread and past the URL into there and click OK.
edit: here's the easiest way. Upload to photobucket and view the photo, then copy the IMG code (just click in the box where the arrow is pointing and it will copy automatically to your clipboard) and simply right-click and paste that code into a new post: