If you keep photos on a hosting site, which I do, it is easy. I keep my photos on Smug Mug.
On your hosting site: Select "share photo", select size of photo to share. Choose the code for "photo only" and copy to clipboard.
In DW; post in Rich Text mode. Choose the icon with the little mountain on it. Copy your url for the photo into the box it provides.
Your photo should show up in your post.