Below the post (when you first hit new post) you'll see some text boxes with the word browse next to them. Click browse and navigate (on your computer) to where you've got the picture stored. Click open and it'll put the path the file in the 2nd text box. In the first, the far left one, you can put the name of the picture. Then click on send and it'll show your picture in the post. I don't ever click the 'store original' box.
Also you will want to make sure your picture isn't too big first. I make it about 500 pixels or so wide and under 100 kb. If you need info about how to reduce image size and file size post and someone (or me) will help you.