In order to post a picture you first need to host it on a web server somehwhere. I have my own server space from my ISP but there are several free photo hosting sites.
Once you have it hosted on a server you need to use HTML to display it in the post. After you write your post check the Edit HTML box in the top right corner of the text body window and it will display your post with all the HTML tags. Put in the HTML code to display a photo from the hosting site. The code is:
<IMG SRC=http://(url of your photo here)>
Then uncheck the Edit HTML box and will see how the post will look with the image showing.
Hope that makes sense.