Add custom text before all image captions in WordPress posts

You can filter the image caption using the wp_get_attachment_caption filter:

add_filter( 'wp_get_attachment_caption', 'wpse422651_filter_image_caption', 10, 2 );

/**
 * Filters the image caption.
 *
 * @param  string $caption       The image caption.
 * @param  int    $attachment_id The attachment ID.
 * @return string                The filtered caption.
 */
function wpse422651_filter_image_caption( $caption, $attachment_id ) {
  // Only run if the caption isn't empty and the attachment is an image.
  if ( ! empty( $caption ) && wp_attachment_is_image( $attachment_id ) ) {
    return 'Photo: ' . $caption;
  }
}

References

• wp_get_attachment_caption filter
• wp_attachment_is_image()