Now that I understand that it's an exercise in changing the tense to past perfect continuous and it doesn't matter if it makes sense, you can say this: She had never been seeing such beautiful painting before she visited the Sistine Chapel. It sounds very awkward and I can't imagine a native speaker ever saying it this way.

Caleb, While she was visiting the Sistine Chapel she saw such a beautiful painting for the first time.

Yes, I need to change the tense

Do you want to change it to past perfect continuous? If so, why?The tense is good the way it is. It wouldn't make sense in the past perfect continuous. The only corrections I would make are: She had never seen such beautiful paintingS(or such a beautiful painting) before she visited THE Sistine Chapel.