You are on the "C Dory for sale by owner" toward the bottom of the forums: The first forum under C Dory Market Place.
There you have a subject and discussion as you got to this above post. The bottom of that window, has "File name"-pick a file with a photo--should end in JPEG", "PNG" "TIF" or similar photo compression file. Click on your file.--then put a description of the file/photo, in the "file comment" section--then "preview" to view the file, and be sure it is what you want--finally click. "submit".
The most common error is not having the file in a proper format, or having an add blocker.