SELECT `image_ID`,`ID`,reddit_rank(`created` ,  `ups` ,  `downs`) AS rank
FROM captions s1
WHERE `image_ID` NOT IN
    (SELECT `image_ID` s2, FROM captions
     WHERE s1.`image_ID` = s2.`image_ID` and rank(s1.created, s1.ups,  s1.downs)  > rank(s2.created ,  s2.ups,  s2.downs))
ORDER BY rank DESC

captions:

ID  image_ID    created     ups     downs
---|---------|------------|--------|---------
1  | 10      | 04/03/2009 | 1000   | 399
2  | 11      | 04/03/2009 | 5600   | 244
5  | 12      | 04/03/2009 | 343    | 555
3  | 10      | 03/03/2009 | 4554   | 300
4  | 11      | 03/03/2009 | 100000 | 200
6  | 12      | 03/03/2009 | 777    | 500
7  | 13      | 24/12/2008 | 1      | 600
8  | 13      | 01/01/2009 | 0      | 700