"My dear friend @markseliger teamed up with @radvocacy and @christiesinc to auction 25 of his portraits – including mine ☺– for COVID-19 relief," she captioned the post.
"100% of sales proceeds of this portrait will go to @NAFClinics, an organisation which provides free coronavirus testing and care nationwide to the medically undeserved."
Seliger told the auction house that the first time he met Aniston, he knew she was destined to be a big star.
"It was uncanny – you thought you knew her, or at least wished you knew her," he explained.
"I was given an assignment to photograph the cast of Friends and they were still a little stunned at how quickly the show had taken off. There was a wonderful stylist, a massive ball of energy, named Chris McMillan who was responsible for the famous "Rachel" cut."
"Since then we've worked together many times – she's incredibly loyal and still as lovely as the first time I met her."
Besides Aniston, 25 other celebrity portraits of Seliger's will also be auctioned off, including images of Leonardo DiCaprio and Oprah Winfrey. Proceeds from each sale will go the charity of the star's choosing.