Using yoga pose pictures to improve technique is a question of careful observation and mimicry; yoga pose pictures can also give details about pose variations that may not be clear from simple descriptions. Using a mirror to check on poses as they are performed can help to perfect one's practice.
Each of the formal yoga positions, or asanas, has specific physical and mental benefits. The best yoga pose resources not only provide photos of the poses, they also list variants, names, technique tips and the benefits of the pose. It's also useful to choose a resource that notes which areas of the body the pose focuses on and which gives the user an idea of the difficulty level. Some resources also provide tips on how to place the poses into a sequence for ease of flow and maximum effect.
Most reputable sites suggest that yoga be learned from a teacher initially to ensure that the learner does not attempt poses that are too hard or otherwise sustain an injury. However, after an initial grounding, many people prefer to develop their practice in private. This is where a picture library or other resource list of poses can be useful, to provide ideas and techniques for new poses and sequences.