Please complete the following steps:
1) Go to the plugin settings page;
2) Open "Custom Code" tab, mark "Activate custom PHP code" checkbox in "PHP" section and insert the following code:
add_action( 'after_setup_theme', 'theme_setup' ); function theme_setup() { add_theme_support( 'post-thumbnails' ); }
3) Save changes.
This article related to Galley by BestWebSoft plugin
Additional resources: Gallery - User Guide
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