I didn't notice that it's the solution of NEXT theme,so I cannot guarantee the answer is still valid.
Original answer:
I also tried to do that before,but failed many times.
Eventually I got the answer in Official Reference Document.
To be brief,use the format below to insert your imgs.
{% gp [number]-[layout] %}


{% endgp %}
[number] is the amount of your imgs.
[layout] is the index of the designed layout. (you can see specific
layouts in the Document)
P.S. The Document recommends enabling the "fancybox" plugin,which I did.
So I am not sure whether it would operate normally without enabling fancybox.Please notice!
For example,you can get:
"image1 image2" (you mentioned in your question)
by using the code below:
{% gp 2-2 %}


{% endgp %}
Hope my answer would be helpful! I kown it's really a struggle to grope alone. :)))