Unfortunately, the dynamic content thing doesn't apply, it was fixed in 2.7 (I checked my config anyways hoping that was the answer for some other caching issues, but those lines were fixed)... and it wouldn't explain these 2:
http://flowergame.net/img/bg/body_bottom.png
http://flowergame.net/img/bg/body_top.png
(it actually seems none of the images were cached, but I only noticed the 3 because they loaded slowest)
Although this does explain it (today's redbot header):
Date: Mon, 23 Nov 2015 13:42:15 GMT
Expires: Wed, 23 Dec 2015 13:15:12 GMT
Cache-Control: public, max-age=2592000
Because Squid is HTTP/1.0 it doesn't do Cache-Control but uses the Expires... same with older browsers and a lot of proxies... But the real problem here is (and I can't believe I only just now noticed it, but I was only paying attention to the weekday):
"Many HTTP/1.0 cache implementations will treat an Expires value that is less than or equal to the response Date value as being equivalent to the Cache-Control response directive "no-cache"."
So you are preventing Squid (and anyone using HTTP/1.0) from caching pages because the 2 dates match... and even if the 'Expires' said Wed, 25 Dec 2015 that would still mean HTTP/1.0 could only cache it for 2 days, which I don't think is what you want. Also - the day of the week doesn't match the date in 'Expires', don't have any idea what kind of problems that can cause, but from that I assume the 'Expires' is supposed to be 2016.
Also, if you could use '304 Not Modified' on the flower images that get posted in the forums that would also help a lot since 'Cache-Control: must-revalidate' means nothing in HTTP/1.0 and those images don't cache for more than a couple minutes...