A little complicated from Facebook, but I'll try to explain.. the other option - I see you have your own domain.. you could load the image there and put use that one:
it would end up something like:
From Facebook, bring up the large view of the image..
If you use Firefox browser, they make it easier.. right click the image and choose copy image location
If you use Imternet explorer, righ click the image and choose properties.. here is where it gets wierd though
using your mouse, highlight the sectiom that says Address (URL) - starting with https:// pull your mouse down till you have highlighted it ALL.. all the way to the .jpg part. You wont see the .jog part at forst, but when you start to drag the mouse down you will.. when its all highlighted, right click choose copy.. its really long and strange looking
Now come back to the post and right click PASTE in the Image popup box..
this is one from you FB page