You can change the width of that image.
Figures won't center correctly (in my vanilla code, if you edit it and it works then go for it) so you'll have to use centered text in place of a 'figcaption'

Any additional text you want on the first page can go here. The gallery styling will auto-place your images based on screen size and how equal the columns are (try making this window bigger and smaller!) BUT that means trying to get an image at the top of the second or third column is pretty tricky. Keep that in mind.

To see some other gallery options, go here.