There are only two reasons for this cause.
- Best news blog theme is using image size minimum 1200 by 600 px for feature image . Hence you need to use at least image of size 1200 by 600 px if you want to set post / page featured image. If not used then your image display will be not in correct way as per demo.
- Since many user has already created post before using Best news theme. In this case, please use regenerate thumbnail plugins to regenerate thumbnails after activating Best news. If the post is created after this theme activated then this step is not required. Please see Video demo about how to create post. https://www.postmagthemes.com/docs/video-guide-of-new-blog/how-to-create-new-post/