When the car sits, the converter can (and usually does) drain into the pan, so it adds a few extra quarts in there which may cause it to leak more than when it is running. Still I think the #1 thing to do is change the shift shaft seal, from the pictures you posted, it is definitely leaking and it will leak when the car sits too. The gasket you have is about the best one to use. Again, start with the shift shaft seal, then after that is done, clean it again and see what else may be leaking.