Mod_pagespeed and resizing large images

Certain large images are not being resized by mod_pagespeed. I get a filename like


so some optimisation happened, but it’s still full resolution. Other images do get scaled down and get names like 600x400xfoo.jpg.pagespeed.ce.4zN0QRGZVI.jpg

I tried setting /etc/php53/conf.d/php.local.ini to have

memory_limit = 256M
upload_max_filesize = 64M
post_max_size = 64M
max_execution_time = 500
max_input_time = 500
log_errors = 1
error_log = /home/unhammer/php.log

(and a phpinfo() call confirms it’s been set correctly) but still no changes to the large images. Is there some other option I have to set, or do I have to ensure all images uploaded are max N pixels large (for some N discovered by trial and error)?