There are only two reasons for this cause.

• New blog theme is using image size minimum 1440 by 600 px for feature image . Hence you need to use at least image of size 1440 by 600 px if you want to set post / page featured image. If not used then your image display will not be in a correct way as per demo.
• Since many users have already created post before using New blog theme, hence please use regenerate thumbnail plugins to regenerate feature image required by new blog theme after activating new blog. This step is required even you have meet above criteria or close to achieve it.