Read this: https://codex.www.ads-software.com/Post_Thumbnails
The default image sizes of WordPress are “thumbnail”, “medium”, “large” and “full” (the image you uploaded). These image sizes can be configured in the WordPress Administration Media panel under Settings > Media.
The default Post Thumbnail image size can also be set by your theme in functions.php. Look for something simular to this:
if ( function_exists( 'add_theme_support' ) ) {
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 150, 150, true );
}
https://codex.www.ads-software.com/Function_Reference/set_post_thumbnail_size
https://codex.www.ads-software.com/Function_Reference/add_image_size
delete something similar to this from your single.php to get rid of the post thumbnail on single post pages:
<?php
if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.
the_post_thumbnail();
}
?>
