Yes, it is possible to change the order of images list, for this follow the next instructions –
1. Open the Images List of the gallery you need.
2. Point on the picture with the mouse and you will see draggable arrows – http://prntscr.com/gpee8p
3. Click left mouse button the drag this picture to a new place – http://prntscr.com/gpeffy
4. After your place pic on the needed place you will see an information box in the right up corner – http://prntscr.com/gpefw3
5. To see changes on the the front-end – please clean cache and cookies in your browser and clean cache in your cache plugin and reload the page with the gallery.
In the case you still can’t update the placement of the picture, please provide the page link on your site with the gallery. Thus I can see what’s wrong and help you.