How to allow visiting commentors to add images?

(Have searched Codex, Forums, wp-plugins, haven’t found an answer)

How can I allow visitors to post an image into my comments? I’d like to allow them to insert an absolute URL to an image hosted on another site, but WP seems to strip it out of the comments before it’s published.

I haven’t had any success finding a plugin that can help or any info anywhere, really. Ideally I’d be able to toggle it on or off for each post, but that’s probably pushing my luck.

Is this possible with WP?

Theme is custom made, if that makes a difference.

Thanks for all help.