1) This theme is utilise featured image as WordPress standard, if you wish to use another method to fetch image like grab first image of your posts content, it definitely need custom change to achieve it. You may need hire freelancer to help you out, or you can use our custom change service actually.
2) Do you use hardcode styling on your posts? Can you provide your blog url so I can check it directly?
3) If you just import posts or use this theme from previous theme, you may need to regenerate thumbnails to refresh image size.