“if physicalism is true, she would know; and no great powers of imagination would be called for”. 
The first response to this is to ask what Jackson means by the word “knowledge” (or by the words “knowledge of red”). This seems like an odd use of the word ‘know’. How would Mary, or anyone else, know what red is like? What is the epistemology of knowing red? Even if Mary could sense red, how could she also know red (or what red is)? Could she, or anyone else, be wrong about red without inter-communal responses?
ii) then Mary would know what red is.
knowledge by description = knowledge that
However, according to our discussion, neither knowledge by acquaintance nor knowing how are, in fact, examples of knowledge (strictly speaking). Only knowledge by description and knowing that are true examples of knowledge. Alternatively, perhaps we can't have one without the other. That is,
i) We can't have knowledge how without knowledge that. Or,
ii) We can't have knowledge that without knowledge how.
Mary needs knowledge that red is red, as we said, before she can learn how to distinguish red from other colours. Alternatively, she must know something in order to know that it's a colour or that she has had a new experience outside her room. To know that red is red, she must know how it looks. How does this work for the other distinction? Thus:
i) We can't have knowledge by acquaintance without knowledge by description. Or,
ii) We can't have knowledge by description without knowledge by acquaintance.
Lewis, David. 'What Experience Teaches' (1990).