Please always use the featured image with high quality. In our theme we recommend you to use “Acme Fix Images” plugin to make the size compatible with theme. Please follow this documentation for making the image size:
1. First setting up images size on media: https://doc.acmethemes.com/supermag/#SettingupImageSizes
2. Use the plugin and fix the images:https://doc.acmethemes.com/supermag/#Howtomaketheimagesizeasondemo?
Please follow the above instruction and try once.