I researched this issue a little bit, including discussions with Actual Copyright Attorneys, a few years ago when I wanted to assign copyright on some stuff I'd written to the public domain. (Apparently it worked, as some text from that material has wandered into a Wikipedia entry.)
You *can* transfer your copyright to someone else, but that means transferring *all* the rights, so it becomes legally as if the recipient had created the work themselves. It doesn't sound like this is what you want to do. As others have suggested, you want to deliver the photos to your friend with an explicit licence spelling out what rights you're granting to her.
Unless you think there's a real chance of the situation giving rise to a formal legal dispute, a document stating your intentions in clear language should do the trick. If you really want to make sure that you're covered in case you have to go to court over the use of these images, you need to involve a copyright lawyer in drafting the licence, but from the first message it doesn't sound like this is likely---you just want to make sure there's no confusion about who owns what, among parties who can all be expected to act in good faith.
In your position, I'd write down what you mean---"I'm granting you the rights to do X, Y, and Z with these images, and I retain the rights to do A, B, and C"---have all parties involved sign off on that document, and feel pretty comfortable with it.
-NT