@v@no: I just ran your sql sentence and I get this result:
# Query_time: 9 Lock_time: 0 Rows_sent: 10 Rows_examined: 730943
SELECT c.image_id, c.comment_id, c.user_id as comment_user_id, c.user_name as guest_user_name, c.comment_headline, c.comment_text, c.comment_date, i.cat_id, i.user_id, i.image_name, i.image_media_file, i.image_thumb_file, u.user_name as user_name, s.user_name as comment_user_name
FROM 4images_comments c
LEFT JOIN 4images_images i ON i.image_id = c.image_id
LEFT JOIN 4images_users u ON u.user_id = i.user_id
LEFT JOIN 4images_users s ON s.user_id = c.user_id
WHERE i.image_active = 1 AND i.image_allow_comments = 1 AND i.cat_id NOT IN (0) AND i.cat_id NOT IN (0) AND i.cat_id NOT IN (0)
ORDER BY c.comment_date DESC
LIMIT 0, 10;
What bothers me? Rows_examined: 730943
Do you have any solution for this kind of problem?