Any money received for property that you sell is capital gains if you sell for more than the cost basis (no matter who the buyer is). In the case of a gift, the cost basis is the same as the cost basis of the giver. That could be hard to determine. If the painting was given to your friend by the painter, then the cost basis is the cost of the paint and canvas, even if the painter is famous. If the friend bought the painting, the cost basis is what he paid. If the painting was inherited by your friend from a previous owner, the cost basis is the fair market value on the date of the previous owner's death.
Hopefully you can establish provenance that includes costs at various points of ownership transfer.
You are required to report the gain as taxable long term capital gains on schedule D. The tax rate is 15% (or 20% if you are in a very high tax bracket.) The buyer may or may not issue you a 1099-MISC to report the payment to you.
If you are audited, and can't prove the basis you claim, the IRS is not required to give you credit for any basis, so do the best job you can to document the cost basis so you can report a lower gain.
