Slider images are already centered, you need to make sure your images are wide enough to fit the theme (1152px) otherwise they will float to the left if they don’t fill up the slider. 聽This is just the way the Genesis slider is built.
For featured pages and posts you can look for this (in your css) –
`#home-feature-bg-alt .featuredpage .page,
#home-feature-bg-alt .featuredpost .post {`
you can add your code and center it from there.