Responsive design using CSS

I found difficult the centering process, whether I wanted the image to be centered in the bottom, center, top or any other place. What do you recommend using in that case?

How do I know what is the exact size of something I'm creating from scratch?

How can I use less code to do more? And I ask this question, because when comparing with others' codes, I noticed they used substantially less codes than me to do everything I wanted to. Like centering, sizing, moving, etc.

Thanks in advance for any possible feedback. I would really appreciate your help to keep growing.