If you can, try to get hold of IRCs (international reply coupons). Keira's agency will be able to exchange them for stamps, so that sending you back a signed photo won't cost them anything. That said, when I sent my stuff back in 2003 or 2004, I didn't add IRCs or a self-adressed envelope, and I still got the three things I sent back, and an extra photo. I would assume Keira receives a lot more fanmail these days, though.
Also, you probably won't get a handwritten reply from Keira - as far as we know, this hasn't ever happened. If you send photos with your letter, she may sign them. If you're lucky, they may add her standard headshot photo with a signature. Don't count on a letter, though