You have to have your pictures on the web somewhere ...
1 - go to your picture on the web
2 - right-click on it and "Open Image in New Window"
3 - copy (Cntrl + C) the address in the top address bar of that new window
... then, when you click on the
button, you'll see this button above the text box:
... click on it and paste (Cntrl + V) in the address to your picture.
... when you submit your post, you'll see your picture.
I hope that helps.