Learning words by translation is fine but what the book argues is that it's very inefficient, because you seed them with respect to your original language so you build a habit of going back and forth between the languages when trying to come up with a word instead of staying immersed.
The advantage of images is first of all that visual cues are very powerful for memory, the more senses you associate with a memory the stronger it will be (I wonder if anyone has ever tried to incorporate smells into SRS?). Furthermore, it is not always easy to find a decent image but the mere search for this image will make your brain work with that word in mind and create associations.
Granted, it is not easy to find images for words such as "philosophy" but with a bit of creativity it is possible and if not, it's always possible to explain the target word in the target language to stay immersed.