Thx Darklinki, now I understood how it is supposed to work and because of that understood the Construct 2 documentation.
So the solution to this problem is:
and then store CanvasSnapshot as value in the webstorage under whatever key you like. And later on sprite -> load from URL: webstorage.localvalue("whateverkey")
But I have another question: is it possible to reduce the size of the screenshot before I store it in the webstorage to save space, since it will be displayed as small version in the load menu anyway?