Arnold Schwarzenegger -- Alleged Sex Photo Already Fetching $150,000+

A photo of Arnold Schwarzenegger caught stark naked in the throes of passion is worth roughly $150,000 -- AT LEAST -- so says a really popular porn site ... and it wants to buy the pic RIGHT NOW sight unseen.

TMZ broke the story ... a financial entrepreneur named Jeremy Frommer purchased storage lockers once owned by deceased Penthouse founder Bob Guccione -- and found a treasure trove of historic erotica, including unpublished nude photos of Madonna, Lauren Hutton, and Arnold Schwarzenegger.

Now, YouPorn.com wants to acquire the pics -- in order to publish them online -- and has already sent Frommer a letter, offering $150,000 for the whole lot.

YouPorn is specifically interested in the Schwarzenegger pic, which reportedly shows the former Mr. Universe nude in the act. YouPorn writes, "Let's face it. The real value here is in Schwarzenegger's 'Schwanz.'" And you don't need a translation to know what that means.