Or, if you have the picture on a hosted site, like Flickr or Imgur, copy the image address, which will look something like this: https://this.is.a.picture.jpg
Go to the menu bar of your post, and click the little icon that looks like a framed picture.
In the Insert Image pop up box, select the From URL tab, paste the copied address into the empty box under URL, and uncheck the Retrieve remote file... box.
Hit OK