Centering a GIF in a gallery post

Hi,

I am really new to website making.
I imported a GIF image in a gallery post on my website. The GIF is smaller in size than the rest of the images in the gallery, and it shows up aligned to the left of the screen, I would like it to be centered. Anyone can help with this?

I was given a css code to change that by the GraphPaperPress support but I’m not sure where to put it they also said slideshow, and what I’m doing is not in a slideshow, it’s a post with a gallery within it where you click left and right for the next and previous image.

They said:

You can center the images in the slideshow using this CSS code:

.flexslider .slides img { margin: 0 auto; }

If anyone could direct to me how and where I would insert this code to try it out, it would be great.

-Sofia