You need to open a photobucket account for free (
www.photobucket.com)and upload your photo there.
Once you do that you can go to Photobucket and copy the link by scrolling over the picture and copying the link.
Then come back to the campfire and hit Reply to my or anyone's message, next click on the third button from the left just above this text box, a pop up window will appear magically.
Paste the link over the text in the pop up box. Voila! You have posted your picture.
It's a wonderful pain in the ass the first time but once you know how it's a piece of cake.
Good luck with your sale.