This actually pretty easy.

If you just upload a bunch of pictures and write some text, the FIRST (uppermost) uploaded picture will appear at the top of the article, and the rest will be presented as a collection of thumbnails at the bottom.

Altering this behavior is easy by using picture tags in your text.
Inserting a picture tag is as easy as typing [picnn], where nn is a consecutive number of the picture in your lightbox. Please remember to use square brackets. If you look carefully in the lightbox, you will notice that every uploaded picture is presented with its corresponding tag as a reference.

Now on to the practical application:
To use picture no. 10 as a heading photo in your article, just write

--- pic10 not found ---

at the beginning of your text.

If you want to insert picture no. 5 within your text, just type

--- pic5 not found ---

where you want it to appear.

Whenever you use picture tags in your text, all unsued pictures will still be appearing as thumbnails at the end of your article.

Easy!

As a final note: The teaser image which will advertise your article in the Headlines will always be the first uploaded picture - in other words,

--- pic1 not found ---

.