score:3
The painting was sold for about $63,250 in 2018 Dollars, and the exchange rate was about 5.2 Francs per Dollar.
I stumbled upon this site containing exchange rates data for the Japanese Yen from 1893 to 1926, and it thankfully includes both the French Franc and US Dollar.
In 1912, one Yen equated to about 2.6 Francs, and 100 Yen equated to about 49.5 Dollars. 1 Yen = 2.6 Francs = 0.495 Dollars, which means 1 Dollar in 1912 was about 5.2 Francs(this is about the same as the 'historicalstatistics' estimate).
So, 12,650 Francs is about 2,432.70 Dollars in 1912. Assuming your $1 → $26 estimate is correct the painting was sold for about $63,250 in modern bucks.
For perspective of what 'the most expensive painting' at the time really means: $2,432 was roughly the cost of a decent house, or about 120 weeks of wages for blue-collar workers(like bakers or carpenters).