I think he means the headline in comments.
I'm curious about this to. Allot of people don't want to change the headline.
I think I know a way, one moment:
Ok found it, it's a bit of an other way then you said but it works:
in template folder / comment_form.html search for:
<input type="text" name="comment_headline" size="30" value="{comment_headline}" class="commentinput" />
Replace with:
<input type="text" name="comment_headline" size="30" value="RE: {image_name}" class="commentinput" />
This way, the headline in the form will always be RE: (image name) unless the user changes this.