You need to check your theme's functions.php as that defines the initial default image type used. Then I have an image type override setup in the plugins config.php - there are notes in there explaining it.
The forum uses a number of icons and you may have a need to use a mixture of image types. Setting this setting to true will invoke a function that will first check for an image of the type set in your themes function.php $_IMAGE_TYPE. If the icon of that image type is not found, then it will use an image of type specified by the $CONF_FORUM['image_type_override'] setting. Set $CONF_FORUM['autoimagetype'] to false to disable this feature and only icons of type set by the themes $_IMAGE_TYPE setting will be used.