Prestashop 1.5.4.1 product images displaying question mark

Prestashop 1.5.4.1 product images displaying question mark

In my prestashop website, all product images are showing only question
marks. This happened for the default theme as well as the theme I'm
currently testing.
I tried rebuilding thumbnails which didn't help. I've tried disabling all
the cache options, and enabling them again. No luck so far. All the
solutions I've been able to find seem to apply only to older versions of
prestashop. One example suggested adding a chmod line in images.inc.php
which made sense to me, but my images file doesn't include the lines they
suggested putting it above.
http://kb.siteground.com/uploaded-prestashop-images-are-not-displayed/
My website is http://printbot.ca
When I view what image is supposed to be displayed, one URL is this:
http://printbot.ca/27-large_default/1kg-black-pla-filament-175mm.jpg This
is on the black PLA product
http://printbot.ca/175mm-pla-filament/8-1kg-black-pla-filament-175mm.html
When I go to that JPG, it shows up as a question mark, not a broken link.
I can't find the file in my file system.
Important note though, all the images show up correctly in the back-end. I
can see them in the product creation page and view them when I open the
product to edit it. The correct path to the image is
http://printbot.ca/img/p/2/7/27.jpg
Thanks for any help you can offer!!