I can only guess – but it was probably an update to something tha contained the images. If you didn’t have any featured images, then I guess something in the content was printing images. It could have been the slider or something similar. Those plugins may have been updated, the output or the shortode loading method probably changed. The search did not change, neither did the configuration, and since I cannot see any image tags in the post content, I suspect that one of the plugins may have been updated.
Setting featured images will bypass this, and will work regardless of the content of the items.