Use the Addtional Options to the bottom left of the post area. Click the Browse button and navigate to where you have the picture store on your hard drive. Hit Post.
If you have the picture on a web server just past the link into the post, highlight it with you cursor . Above the smiley faces there is a tab three spaces from the left beside the Facebook icon. Click that tab and it will enclose your link with [ img ] [ /img ]. Hit Post.