What does it mean when a pic turns green?

This morning, when I click on certain thumbnails, the pic appears for a second and is then replaced by a plain green oblong. Is there something weird in my system, or a bug in the site.

For whatever it might be worth ... I just spent an hour or so on the main site, and everything was working just fine for me.  No green pics to be found.

Whatever it was ... it seems to have cleared itself up. Hopefully permanently.

I have not made any changes recently that would do this.