Why is "xiang4" in 摄像 and 照相 different from each other?
A minor correction, it should be xiang4 instead of xiang2.

When we are talking about photography, 相 typically refers specifically to a photo, while 像 usually refers to visual representations in general (be it still, like 图像, or moving, like 录像).

摄 translates to "capture", and therefore 摄像机 is a device that capture images (that aren't necessarily photos). On the other hand, 照相 can only refer to taking photos. So 相机 (the abbreviation of 照相机) is a device that takes photos. The choice of words here mainly describes the main function that the device is designed for.