Thank You so much for your interest on our theme SuperMag.
1. We recommend you to use SEO By Yoast plugin, and it help you to make the image size while sharing the content from Facebook and other social media.
2. You mean to say, you want to remove that No Image Available or want to replace it ? If you want to replace the No Image Available with your own image, you will get that option in the premium version of SuperMag i.e. SuperMagPro.
If you want to hide that section, you have to do some custom Work for it. It need some theme customization.
3. If you want to use another slider in place of featured slider, you need to customize the theme, because this feature is not available on the theme.
Note: Child Theme is recommended to customize theme.