That’s right, you can unhook it, and then hook your own function in – it should work in a child theme or your own plugin.
For example:
remove_action( 'generate_before_content', 'generate_blog_post_image' );
add_action( 'generate_before_content', 'generate_blog_custom_post_image' );
function generate_blog_custom_post_image()
{
// Your function in here
}