I cannot get thumbnails to display in pages since upgrading to 17.2 (and even in 18.2 now). If I click on the "JPG" image icon put in place of the thumbnail (or the alternate text that has a link), I see the image just fine. If I do a direct display via the URL, I see the image just fine. Just will not appear in a page. See http://h600.org/wiki/Half+Identical and http://h600.org/wiki/B10DNA as two examples. I have other sites on the same server with the same issue.
I have been pulling my hair out on this one. Just not sure how to debug the problem it seems. Have read other forum posts from earlier releases (e.g. see Problems with thumbnails since upgrading to 10.1) but not found anything useful. This started when I upgraded to 17.x from 12. And continues (possibly worse) with 18.2. These two upgrades are the first that forced an override on the server to use PHP 5.6 and I suspect this has something to do with it.
All the images are stored in a gallery. Images are in the file system, by default. None in wiki_up. All in a non-accessible directory outside the WWW URL access. None have permissions to restrict access. (for the pages referenced above)
I have cleared caches; both on the server and on the browsers. I have not re-uploaded images (or resaved them) that I am aware of. But have been trying to figure this out for months now. Using different browsers on different devices as well. When I come in anonymously on a clean browser, I see no images on either page.
I do notice that it sometimes works slightly better when logged in than when accessing as anonymous. But it also behaves differently as I progress. Making me think it is a cache issue. Neither page referenced above used to display anything (or will on a fresh browser with cleared history / cache and using anonymous). The B10DNA page seems to display most, if not all, image thumbnails while I am logged in. Even if I clear server caches but not if I clear browser caches.