Readers familiar with The Lord of the Rings are either going to have their own "picture" of Éowyn in mind, or they're going to be imagining Miranda Otto.
With all due respect to Ms. Otto, a lovely woman and a fine actor, the Éowyn I'm picturing in these stories needs to be a little bolder, fiercer, saucier, and...this is important...more overtly sexual. As such, the mental image I've carried throughout the writing is Katheryn Winnick's version of Lagertha from Vikings. She's decidedly curvier than the Éowyn in the books (and in my story), but everything else is pretty much exactly what I'm picturing.
Issues of costuming and hairstyling aside, I mean this, or this, or in more peaceful moments this.