Center of 2 column

Hello
I put the plugin and it is perfect, exactly what I wanted.
But I would change the presentation: or center images or put two column like this: http://img15.hostingpics.net/pics/837939Sanstitre.png. I look at the files but I do not know what to change
thank you
Best regards

https://wordpress.org/plugins/really-simple-gallery-widget/