Latest iteration of patch attached.

> In theory, we might not need to clear image cache as long as we call
> `image-flush' in `org-link-preview-clear'.

Removed (clear-image-cache) when refreshing since we run (image-flush)
unconditionally in org-link-preview-file.

> My hesitance about top-level require is that ol.el is loaded by default
> and that the preview functionality may or may not be used by the
> users. So, loading image.el ought to be optional.

No change here.

> It means that you are scheduling every single preview batch to fire at
> the same time. I think that the timers here should run sequentially -
> (1) fire the first batch without delay; (2) wait org-link-preview-delay
> and fire the next batch; (3) ...

Done, and tested preview run timings to be sure.

>> +           (lambda (previews)
>> +             ;; (message "queue: %d" preview-queue-size)
>> +             (cl-decf preview-queue-size)
>> +             (dolist (preview-spec (nreverse previews))
>> +               (when-let* ((ov (cadr preview-spec))
>> +                           (buf (overlay-buffer ov)))
>
> ov or its buffer may not exit at the time the timer is fired.
> Because, for example, the buffer is killed, or because user edited the
> underlying link before it got displayed.

Handled by (overlay-buffer ov), as explained in the previous email.

Karthik