Gary, you have to add one picture as an Attachment. Right below the very message box you type in your replies, there's another box, "Additional Options". Hit the "Manage Attachments" button down there and upload the picture you want to add as a thumbnail from your computer directly to PlanetArmor. Now you're all set. Just click on "Submit Reply" (or in this case: Edit Post) and the thumbnail appears magically
I hope this helps.
The Staghound looks great so far. Looking forward to the next updates.
Johannes