[Logician]

It requires a hack and here it is:

First create your image and put it into yourboard\images\ directory.

Then edit admin\functions.php and find:

PHP Code:

if($dobbimagecode and ($bbuserinfo[userid]==0 or $bbuserinfo[showimages])) { 


BEFORE that line add:

PHP Code:

 if($bbuserinfo[userid]==0) {
      // do [img]https://vborg.vbsupport.ru/[/img]
      $myimagelock='onlock.gif';
      $bbcode = preg_replace("/(\[)(img)(])(\r\n)*([^\"".iif($allowdynimg,"","\?\&")."]*)(\[\/img\])/siU", "<img src=\"images\\$myimagelock\" border=\"0\" alt=\"Register to View This Image\">", $bbcode);
    } 


Replace "onlock.gif" with your new image (be careful about the caps of file name!) and save your file. That's it, now if the forum viewer is a guest, all images will be displayed to him as your custom image while other board members will be ablo to see them as they are. (One downside: This hack will convert images in user profiles too)

BTW I liked your idea, can be useful for some people..

Enjoy..
Logician