For some reason on Magento 2.1.5, our category pages display the product name and link to product, however the image is not showing. If you open the product the images are there.
I'd like to check the following things:
I'm don't have time to check a code, but looks like from last magento updates images doesn't create when page is processing and you need cron is running to generate resized images
Also you can run command from command line:
$ bin/magento catalog:images:resize