How can I center a views slideshow?
You need to use image styles to get all the images the same size. 

From the admin menu, Configuration > Media > Image styles. 

Create a new image style, with a scale and crop attribute to the exact dimensions for your slideshow. Name it "slideshow-image".

Then go back to the slideshow settings on your views slideshow, and select "slideshow-image" as image style.

Now every image will be the same width and height, regardless of the dimensions you upload them in.