Only use the address in the 'Linked Thumbnail' line - Don't use the More Icon -
You have to open a window for the post and a window for the photo.
Copy and paste that address from the photo screen to the post you wish to add it to and it will show up as the address. When you submit the post, it will show up as a thumbnail and when you click on the thumbnail in that post - it will open a separate window where it will be full sized.
Let me know if this works for you.