If you want to insert an image off-site image in the body of your message you need to copy the LOCATION of your image and insert it between the img tags
If you click on the first icon in the second line in your post you will get the tags. Just insert the location between them.
If you want to attach the miages at the bottom of your post use the Attach option under Additional Options.
I can't remember if there is a white list of acceptable locations to post images from