bbPress.org

I think the code is in the wp-includes/js/quicktags.js file on line 621 , and in the quicktags.min.js on line 152 of the formated version.

if ( src ) {
   alt = prompt(quicktagsL10n.enterImageDescription, '');
   this.tagStart = '<img src="' + src + '" alt="' + alt + '" />';
   qt.TagButton.prototype.callback.call(this, e, c, ed);
 }

best regards