4.3: modified IconCreator to use IM instead of PIL by default, if it is ...

...installed
This is true, however at this point we already know we don't have IM to fall back to. If this is the case and PIL still fails, some sort of exception should be raised indicating that the necessary tools aren't installed to handle an image like this one. Would that simply be, "raise TacticException('Error creating thumbnail for image')", or perhaps something more descriptive or elaborate?