The image has to be a current image found on the web. For example, if you have photos stored on PhotoBucket.
1) When you find the image you want displayed in your post, right click the image and select "Copy Image Location"
2) Click the
button
3) This will paste "<img></img>" into your post (except the "<" will be "[" and ">" will be "]"). Click in the middle of the brackets and press "Ctrl-V" or right click and select "Paste"
That should do it.
Fly