In case you are migrating from another theme to our theme, most probably you will experience some differences in the thumbnails sizes, so you will need to run “Regenerate Thumbnails” plugin to regenerate thumbnails in the new sizes.
- Go to Plugins > Add New
- Search for Regenerate Thumbnails (created by Viper007Bond)
- Install and activate that plugin
- Go to Tools > Regen. Thumbnails
- Click Regenerate All Thumbnails button and let the regeneration process finish to 100%
Read more: Change featured image sizes with a Child Theme.