Images not loading, weird CSS class inserted

This is very weird and awkward to explain, so please bear with me. When I insert an image into a page, it sometimes won’t display, in the editor and/or on the live page. This varies by browser too. Seems to show in IE and Firefox Developer, but not in Firefox regular or Chrome. I’m using the WordPress TwentyFourteen template.

Another weird thing is that when the image is inserted and the page published or saved, there is a weird CSS class that gets added to the image tag that is a long string of seemingly random lower case letters, and this class name changes; that is, it’s not the same one every time, but seems to change if I login again (and maybe reload the browser?).

I’ve tried inserting the image from different browsers into a fresh page, but it still mostly displays only in FF-dev and IE, but not Chrome or FF-reg.

I don’t always see that weird CSS class either. I see it most consistently from FF-reg, both in the Text tab of the page and from FF-reg WebDev-Inspector or Firebug. I don’t see it in Chrome but I do see a local style=”display: none !important” added to the tag. IE and FF-dev show no weird extra classes or styles.

Even if I try to disable the style or class from inside WebDev or Firebug, it snaps back anyway. I have no clue where this stuff gets added. It seems to behave like some kind of malware but I don’t know how to begin to find out more, and so far it seems to be more a nuisance than dangerous. I’m running Wordfence and I don’t see anything serious coming up in the scans. I’ve tried disabling all but the most essential plugins (e.g. Wordfence) and there was no change.

An example of the altered tag (in FF-reg) is:

<img 
  class="alignnone size-full wp-image-2462 <strong>wzxehmwknkjwmsonqgze</strong>" 
  src="http://mydomain.com/wp/wp-content/mymedia/banner160x600.jpg" 
  alt="banner image" 
  srcset="http://mydomain.com/wp/wp-content/mymedia/banner160x600.jpg 160w, 
    http://mydomain.com/wp/wp-content/mymedia/banner160x600-80x300.jpg 80w" 
  sizes="(max-width: 160px) 100vw, 160px" 
  width="160" 
  height="600"
>

I only just discovered this today, but it’s probably been close to a year since I inserted any images to the pages of this site, so I have no idea how long this has been going on. When I Google the problem, I’m seeing reports of similar problems from an old update from a little over a year ago, but people mostly talk about checking for potentially interfering plugins, like slideshows or photo galleries. This site has nothing like that.

Any insights about this would be most appreciated! I need to do some work with images and I need to have a clean installation, especially since we want other sites to link to these images.

Let me know if you need more info. Thanks very much in advance!