§140

Pictures don't determine a use. So why are people tempted to think that they do?
- We can understand something in an instant and a mental picture is the kind of thing that might come to us in an instant.

The cube case might lead us to say that although we are not under a logical compulsion to apply a picture in a certain way we must be under a psychological compulsion. This could lead us to distinguish psychological compulsion from logical compulsion. Wittgenstein clearly thinks that there is something wrong with this distinction. I assume the problem is with the idea of logical compulsion.

We might have the same picture before our minds when we hear a word but apply it differently - and a different application means different meaning. Here, presumably, Wittgenstein is emphasising that the use of a word is internally connected to its meaning but a mental picture is not.