Arnold Schwarzenegger -- Sex Photo Reportedly Surfaces

Arnold Schwarzenegger has been thrust into a very belated sex scandal ... because there are reports that an old photo of a "young Arnold" performing a sex act has come to light.

The photo is reportedly part of a treasure trove of artifacts that have been discovered in a storage space owned by late Penthouse founder Bob Guccione.

According to the New York Post, the Arnold pic -- along with a bunch of other VEERRRY interesting items -- were discovered by a guy who went all "Storage Wars" and bought up a bunch of the lockers after Guccione went bankrupt.

Among the treasures ... unpublished nude pics of Madonna and Lauren Hutton ... and Bob's personal files detailing the situation surrounding his decision to publish pics of then-Miss America Vanessa Williams.

It's unclear if the new owner plans to release the Arnold pic.

We reached out to Arnold's rep -- but we haven't heard back.