Ike and Tina Turner weren't at Woodstock.
There was no partial nudity from Janis (at least not while she was on stage) at Woodstock. She wore a long-sleeved tye-dyed blouse and pants, and a long dark vest which she removed about midway through her set.
Pictures of Janis at Woodstock can be seen here, so you can compare the outfit she was wearing with that in your pictures. But if Ike and Tina were there, it wasn't Woodstock.
Janis DID appear onstage with Tina Turner at Madison Square Garden in 1969, but she only performed one song and she wore a large white fur hat.
If you can scan a photo and post it, I'll be able to tell you if it's Janis or not - but I doubt it is, based on your description.