How do I make images open in a new tab by default?

If you cannot write codes:

There is a Plugin MDC Target Blank, that’ll solve the issue. When someone clicks on a link (or an image that is hyperlinked), it’ll force to open in a new tab. It adds target="_blank" to a tags.

Older versions of this plugin had a bug, that’s fixed in new versions. So, don’t worry about the plugin ratings.

If you can write codes:

Add this code to your theme’s functions.php file

function mdc_target_blank($content) {
    $post_string = $content;
    $post_string = str_replace('<a', '<a target="_blank"', $post_string);
    return $post_string;
}

add_filter( 'the_content', 'mdc_target_blank' );