Website not resizing for different displays

To whom it may concern:

I am using the Asteria Pro theme for my personal training website, and a lot of my clients have noticed that the display is different on different screens. When I created this website, I used my macbook pro (13 inch) and it looks fine. This poses a bit of a problem because some of the pictures that will be in the front page slider will be cut off. It would be greatly appreciated if someone could assist me in fixing this problem immediately as this is my company website. Thank you in advanced

ps- Im not the most savvy with css so if possible, please give me the “Dummy version” to fix this! 🙂