There are numerous ways to post photos using urls and links from photos on the internet. If you know the url of the photo you want to post here, use the "IMG" button at the top of the body of the message box. Press "IMG" and [img.][/img.] will appear in the message box. Then put the link of the actual photo that's online that you want to post in between the [img.]and the[/img.].
It'll look something like this:
Then hit the "preview" button to check it out.
The Emmylou photos in the above post were on my computer. The same photo below is from a link on a site where I found that photo. It's posted using the [img] method.