As in so many "theme" threads here, all I see for many posts is:
Linking directly to a picture often doesn't work, as the TGP host page restricts you from hotlinking. It may appear onscreen to you when you first post it as it's in your browser cache (at least I think that's why).
Much better to upload the image as an attachment, upload it to a dedicated picture host such as www.imagevenue.com, or provide a link to the full gallery and mention the picture number.