I'm having a similar experience on an iMac running OSX 10.11.6, and on all browsers on the machine. I looked at the Safari-specific support doc, but this is clearly outside of the individual browsers. Certain pages will load with the layout in place but the graphics missing and a "?" box in their place. It's not the web sites, either, as I can switch to my MacBook Pro laptop, call up the same pages and see everything that's missing. This seems to have occurred overnight about 3-4 weeks ago. One day the sites all looked fine. The next, no graphics. I've gone through System Privacy settings and deleted stored data for the specific sites, too, then opened them again, but this made no difference. It might be something simple and specific in Security & Privacy, but at this point, I don't really know what to try next.