Can I tell her to take it down in her own room?
Can you two work out a compromise before telling her to take it down?
What compromise? I'm telling her to take it down. She's got some nerve
You shouldn't tell others what to do when it comes to decorating their own space. They might understand if its a communal area, but if its their personal space you might be overstepping a boundary and will upset them.
It's her fault for putting that poster up in the first place.
I think it is best to avoid telling others what to do, especially if the issue is not one that is extremely serious.