If you see this theme demo, you may be see there is a section for gallery items.
You can add gallery images form the admin area. Navigate to Gallery > Add New Gallery. Upload a feature image for gallery image. You can add some categories if you want to show these pictures as category based.
The categories itself will look like bellow in the frontend of you site.
Now, go to your page admin area. Pages> Add New > Backend Editor > Add Element > Bright > Gallery. This addon will looks like below.
Finally, the Gallery itself will look like bellow in the frontend of you site.