Unless you do a "photo album" post the picture has to be on a public accessible website. Then while you are typing your post, click on the "Image URL" tab at the top of the edit window (second from the right). Enter the URL of the picture and select the alignment. That should work. The trickiest part is getting the picture on a public site, but your snapfish site should work unless you have to login to see it.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013