Hi Peter,
First off, let me say welcome to the forum. Your contributions so far have been a great pleasure to view.
Posting the full-size images is a little confusing at first. You have to reference them from an outside source, such as Flickr, or some other hosting gallery. From Flickr, for instance (which is where I post from), you choose the image you which to post, then choose "Actions" above the image on the left. From Actions, I chose the size with "View all Sizes", and select the size I want (the max for an image on the forum is 900 dpi on the long side (although some longer get posted)). Then I right-click that image, and choose "Copy Image Address". I then make the Reply I want in the forum thread, and post the image using the little yellow box above the Reply box.
I hope this makes sense. If not, I'll try to clarify. Good luck.
--Lloyd