That's the nub of the issue. If it could be sold for $20m, then there would be tax to pay on the profit. By "donating" it at $20m valuation, they get a tax credit higher than the $4.5m purchase price. This amounts to legalized theft from Canadian taxpayers.
I understand where you are coming from, but situations like this highlight a not unexpected consequence of trying to use the tax system to accomplish goals that aren't related to the purpose of income tax.
To save $4.5 million in tax, the donation has to be valued for tax purposes at a value a lot higher than $4.5 million. And the donor/taxpayer has to have a lot of income subject to tax.
So the system usually results in some of the value of the donation coming from the donor, and some coming from the taxpayers.
If the donor turned around and sold the photographs for $15.5 million more than they paid for them, they would only pay tax on half of the gain. So by deciding to donate them instead, they were forgoing a lot more than the tax they would have saved.
The net result of this is that the Art Gallery of Nova Scotia is now the owner of photographs that have a market value somewhere between $2.25 million and $20 million.
Annie Leibovitz no longer owns those photographs. She has only received half of the $4.5 million she expected for them.
The donor is out half of $4.5 million, and may end up having to pay Liebovitz the other half, if the donor ends up being able to claim a larger tax deduction than what is currently being permitted.
And the Canadian taxpayer is out an uncertain amount, but certainly an amount that will end up being far less than the eventually determined value of the Art Gallery of Nova Scotia's new collection of rather special photographs.