There are only two reasons for this cause.

• Our theme use image of a certain size while setting up the post’s featured image. Such image is then cropped to different sizes to display them in different layout as per theme design. Hence you need to use at least such image size to have your site as perfect as shown in our design. Such image size is prescribed on the respective theme page. If not use the minimum prescribed image size then your site front end display will not be in the correct way as per the demo.
• In normal cases, many users already have created posts before using our theme. In this case, please use regenerate thumbnail plugins to regenerate the correct size image after activating our theme.