I am interested in crime, its origins and it's impact. I have worked for the prison service and with offenders for 20+ years.
However, I do not and never would have a poster of a serial killer on my wall. Why would someone do that? Most people put posters of people they like or admire on the wall. Thus, in your case, I'd be quite worried. Myra Hindley is not to be glorified or admired in any way.
Personally I'd inform her CAMHS worker. I'd also try and spend quality time with your daughter asking why she has Myra Hindley on the wall, what she likes about crime stories etc... Really try and understand and listen to what she's saying.
You also need to tell her dad who the people are that his daughter has pictures of, on her bedroom walls.
I do think you are right to be worried and you can't do nothing. Keep talking to her. This is teenage time,.so a time when people are searching for identity. You really don t want your daughter to start solidifying her own identity as that of someone capable of doing v bad things.
Talk about the victims.of crime, the impact of crime. If you can, try and encourage her more down the route of supporting victims. Bringing offenders to justice etc....