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in /xoops/class/module.textsanitizer.php look in the function called xoopsCodeDecode.
You can change the regexp definitions, for example replace:
$patterns[] = "/[img]([^"()?&'<>]*)[/img]/sU";
with :
$patterns[] = "/[IMG]([^"()?&'<>]*)[/IMG]/sU";
or add a new definition.