You have to work on the same version, otherwise you will run into great troubles. IDML is a very bad idea for a roundtrip workflow, don't do it.
Make a package.
As a CC subscriber you can install CS6 without additional cost, but you should recommend your artdirector to upgrade, as CS6 id not supported forever and she should expect an end of life in the near future.
You should make a package which includes IDML and PDF. And you have to give your art director full access to the parent folder of the INDD, IDML file and the subfolders of the package. Work on only in the packaged file. Keep all workers on that file on the same version.
How do we pass files back and forth? I tried using dropbox sending her IDML files but the photos don't move.
Your CC subscription lets you install different versions, so if your art director is unwilling to upgrade you'll have to install CS6 and use it for the collaboration.
Also, rather than using DropBox to send files you can use it to sync a folder of files, which can be shared with other DB users. That would save considerable time over packaging and sending back and forth. So if you make a folder inside your local DropBox folder and right click it you should get an option to share:
And you'll get this:
Add your art director and she'll see the same folder in her local DB folder after accepting the invitation. Then it's just a matter adding the ID files and assets to the shared folder and your art director will be able to open and edit them. When either of you make a change and save, the folders will be sync'd.