Hayden Panettiere's Final Instagram Post Was Photo Taken 15 Years Ago

Hayden Panettiere's final Instagram post was a 15-year-old memory ... and the man beside her in the throwback snap tells TMZ he was surprised to see it resurface.

Director and photographer Randall Slavin is the man in the photo, and he tells TMZ ... the black-and-white pic was not recent -- it was taken about 15 years ago at a party inside a friend's home.

He says he recently stumbled across the image on an old hard drive and texted it to Hayden ... but had no idea she planned to share it publicly and was surprised when it was posted on her page on July 27.

Randall says he and Hayden met socially many years ago, though they were not particularly close in recent years. He last saw her in passing more than a year ago.

As we reported ... Hayden was pronounced dead Sunday at 36 after a friend found her unresponsive inside a Greenville, South Carolina apartment complex.

Paramedics attempted to resuscitate her, but she was pronounced dead at the scene.

Police said there were no initial signs of foul play and the circumstances did not appear suspicious.

We obtained the dispatch audio, and there's mention of a possible "overdose."

Randall remembers Hayden as "an incredibly sweet, special woman."