There are only two reasons for this cause.
- New blog theme is using image size minimum 1400 by 600 px for feature image . Hence you need to use at least image of size 1400 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 New blog theme. In this case, please use regenerate thumbnail plugins to regenerate thumbnails after activating new blog. 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/