[Zzed]

In functions.php:
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PHP Code:

function bbcodeparse2($bbcode,$dohtml,$dobbimagecode,$dosmilies,$dobbcode)
{ // parses text for vB code, smilies and censoring

  global $DB_site,$wordwrap,$allowdynimg, $bbuserinfo;

  static $smilies,$bbcodes;
  global $regexcreated,$searcharray,$replacearray,$phpversionnum; 


Add the following directly below it:

$bbcode =  preg_replace("/img\]/i", "thumb]", $bbcode);

This will replace all your IMG tags with THUMB tags.