>>15781
There doesn't seem to be any particular name for it. There's a thin argument for it being a fringe type of contraposto, but hunting it out on Gelbooru and Pixiv shows little else particular.

That image itself just has "arms up", and checking through the first couple dozen of pictures tagged with that and showing a similar pose returns alternatives of "arms behind head", "own hands together" and "stretch"/"stretching", as well as one example of "pillow grab" for a similar pose struck whilst on a bed.

Arms Up and Arms Behind Head are probably rich enough veins to satisfy you without too much additional clicking, but if you're really stuck the bigger list of posture-related tags might help (I can't be bothered hunting through it) --- gelbooru.com/index.php?page=wiki&s=view&id=12313