The singer said looking through shots of the group recording the Sound Of The Underground video in 2002 made her feel like she'd been "transported back to that freezing cold shoot in an empty warehouse where none of us could hardly believe what was happening or even knew what to do".
"Happy Anniversary to the song that changed our lives!" Roberts, 37, wrote.
"20 years ago today Sound Of The Underground went to number 1 and stayed there for 4 consecutive weeks.
"Thank you @peterloraine @polydorrecords for these behind the scenes pictures.
"I’ve been sitting here going through them all and feel like I’ve been transported back to that freezing cold shoot in an empty warehouse where none of us could hardly believe what was happening or even knew what to do."
Fans of the group were thrilled to see the throwback images.
One posted: "Oh. OH. Oh, wow. Wow, wow, wow! That's history right there! Thank you for sharing these! GA forever!"
"Beautiful photos!" said another.
"Thank you so much for sharing these memories. Happy 20th anniversary SOTU!"
Somebody else urged: "Keep feeding us Nic!!!"
One fan said it was "so emotional" seeing old pictures of Harding.
"Sarah is missed so much," said another.
Girls Aloud went on to have a huge career after winning the TV talent show, and had a string of hits including Jump, Love Machine and No Good Advice.
Watch: Kimberley Walsh says Girls Aloud may never perform again without Sarah Harding