Fantastic drawings! I'm not at all an amazing life drawer or anything like that, so I don't really think I can offer much in the way of valid critique. There are some pointers people have given me though that I can share with you though :D
1. This one is more challenging when drawing with photos than with the traditional, sit-in lifedrawing class, but it's good to think of the model as a 3D object with volume rather than a 2d image. When it comes to poses with tricky perspective, sometimes softly tracing the volume of the figure before going over the outline helps for proportion.
2. When it comes to longer held poses, it's easy to get lost in the little details, but try not to focus too much on each limb individually. Instead, take your time at the start capturing the full flow and gesture of the pose, and keep in mind how each part of the body attaches to the rest, even when the points of attachment are obscured (either by pose or drapery)
3. I don't know what weight pencil you prefer, but it's always fun to experiment with new materials. Heavier weight pencils (6B is my preference) play nicely with things like line weight. Varying the line weights will give drawings a sense of weight and volume, and are good for indicating things like muscle tone and the fall of light.
I think your drawings are quite good, and I agree with others about visualizing the models in 3D. A brief review of artist's anatomy would be the only other suggestion I can offer. I think we can all benefit by going back to basics from time to time. You're doing great, so keep it up.