I first upload my pictures onto photobucket which will give you links to your pictures. If you don't have an account yet you'll need to create one. It's free so cost is not an issue.
So when you want to post a picture to a conversation, open up Photobucket and then open up the discussion you want to reply in a second tab/window and then make sure you click the "Use rich formatting" link next to the reply button. Go back to Photobucket and the copy the "Image Code" link and then paste it onto the reply.
It's probably hard to understand just by reading but play around with it on the Testing forums and you'll probably find it comes easier to you.