They say a group of schoolgirls were walking up the Foregate Street railway station stairs at around 4.10pm on 16th May.
One of the girls saw a man pointing his mobile phone under the skirt of another girl in the group, as if he was taking photos or filming.
Officers say the man in the images may have information which could help the investigation.
They're appealing for help to trace him, and are asking the public to:
contact BTP by texting 61016
or call 0800 40 50 40 quoting reference 2300056036
Alternatively, Crimestoppers anonymously on 0800 555 111