The picture -- sold by Christie's and Hunt Auctions in New York -- is considered a Holy Grail when it comes to Jackson items ... given the fact that the guy rarely ever signed anything.

Historians say the all-world baseball player was illiterate and struggled to read and write his own name ... so anything with his autograph is considered sublimely valuable.

In fact, his signature is so rare, reps at Christie's say this is the only Jackson autographed photo still known to exist in the world.

The signature has been given a 9 out of 10 grade ... while Christie's officials say the photo -- which features Jackson throwing a ball at spring training in 1911 -- has "minimal surface wear and no mentionable defects."

The picture had previously sold at auction back in 2015, going for just $179K.

Of course, the piece is also super valuable because Jackson is considered arguably the best -- and most infamous -- baseball player of all-time.

He batted .356 in his 13-year career ... but was also banned from baseball in 1921 for his alleged role in fixing the 1919 World Series.