The snapshot feature will do the work if you need to save and load an image that fills the whole window (a Background).
I hope the features to resize the window on the official nodwebkit plugin will be implemented in the near future.
You can use the Unofficial plugin now that I remember.
scirra.com/forum/topic60647.html
It has a lot more options, including the resizing, etc. But I dont know if once you export the game the changes made by the plugin will work. Either way you lose nothing by trying.