她们正在拍照的英文表达可以是 "They are taking photos" 或 "They are taking pictures." 这两种表达都可以表示她们正在使用相机或手机等工具来拍摄照片。

这两种表达都源于英语动词 "take," 意思是 "拍摄，拿，搬运" 等。在这里，它被用来表示使用相机或手机等工具来拍摄照片的动作。

下面是一些例句：

They are taking photos of the beautiful scenery. (她们正在拍摄美景的照片。)
We took a lot of pictures at the beach. (我们在海滩拍了很多照片。)
I'm going to take a photo of you with my phone. (我要用我的手机给你拍照。)
They are taking pictures of each other with their new camera. (她们正在用新相机为彼此拍照。)