Screenshots in error messages are logged by the Post image on error option in project properties > Playback. They are always the entire desktop, all monitors included.
If you want a limited-area screenshot on errors, disable this option and instead handle the OnLogError event (see Creating Event Handlers for TestComplete Events). For example:
// JScript
function GeneralEvents_OnLogError(Sender, LogParams)
{
// TODO - take a screenshot of the needed area
var img = Sys.Desktop.ActiveWindow().Picture();
Log.Picture(img, "Screenshot at the time of the error");
}