[FIXED] Why is the browser not caching my images?

Issue

I am using nginx and Adaptive Images to deliver dynamically sized images based on device resolution. The response headers being set by the adaptive-images.php file are shown below but every time I refresh the page the browser requests the images again. Why is the browser not caching these images? The browser is Google Chrome and it seems to be setting max-age=0 in the request headers no matter how I refresh. I've tried F5, Ctrl+F5 and entering the URL in the address bar and hitting Enter.

Request Headers:

GET /img/photos/p8.jpg HTTP/1.1
Host: example.com
Connection: keep-alive
Cache-Control: max-age=0
Accept: image/webp,*/*;q=0.8
Pragma: no-cache
If-Modified-Since: Wed, 16 Jul 2014 12:01:31 GMT
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 Safari/537.36
Referer: http://example.com/
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8,en-GB;q=0.6

Response Headers:

HTTP/1.1 200 OK
Server: nginx
Date: Wed, 16 Jul 2014 12:08:55 GMT
Content-Type: image/jpeg
Content-Length: 391104
X-Powered-By: PHP/5.4.30
Cache-Control: private, max-age=604800
Expires: Wed, 23 Jul 2014 12:08:55 GMT
Last-Modified: Wed, 16 Jul 2014 12:08:55 GMT
Connection: Keep-Alive

Solution

It turns out that this seems to be a Chrome feature

See this other SO answer for why: Chrome doesn't cache images/js/css

Answered By - conorgriffin

Answer Checked By - Willingham (PHPFixing Volunteer)