To embed them you'll need to upload them to a hosting service (flickr is popular and free) www.flickr.com
Then when you've uploaded them you'll see a 'grab the photo's url' field below the picture on flickr - copy this url, click the 'insert image' button when you're creating your post on here and paste the url you copied from flickr into the box that pops up.
Copy that address and then click on the yellow icon that looks like a mountain-scene postcard in the thread posting window. You'll get a pop-up. Paste in the picture's web address.
When you post your reply you'll see something like this: