She confronted Nicholas Bagshaw after spotting the 52-year-old pointing his mobile phone towards her at an unusual angle while they travelled on a train from Stourbridge Junction station on March 19 last year.
He denied doing anything wrong but she and her boyfriend reported his behaviour to officials who questioned him and discovered other pictures of women, one of whom appeared around 20 times among the photos stored on his phone, Birmingham Magistrates Court was told.
Bagshaw and this victim had travelled together on several occasions and she was "upset, disgusted and shocked" when told of the secret snaps in which she featured, it was said.
Shabeer Qureshi, defending, said: "These women were not recorded in their own homes and when questioned by police the defendant immediately told them about the images on his phone.
"He was very upfront with officers and disclosed the identity of the person pictured."
She said in a statement: "Nick was friends with my husband and we let him into our home. Now I am angry.
"He abused my trust but his behaviour around women had always been odd."
Bagshaw, from Chiltern Close, Dudley, admitted two charges of outraging public decency, was given bail and will be sentenced at Birmingham Crown Court next month.