With regard to skeleton modeling strategy, you will need to provide more details what you are attempting to use the skeleton for. To reverse engineer a master model of the molded housings of that calculator, I see no need for using a skeleton.
You can import your photos into a Creo model to use as a reference when sketching. Ideally you will get photos of a top and side view (orthographic projection views) and import them into the top and side planes in a model. You can then use these as a guide to create the sketches needed to capture the form factor in the Creo part.