I haven't played around with images inside Quest, or worked with Gamebooks yet, but depending on how Quest sets things up you could add the following CSS
#WHEREVERTHEIMAGEIS {background: url(YOURIMAGE) top left no-repeat; background-size: contain;}
Just change WHEREVERTHEIMAGEIS to the element of the id that actually houses the background image, and change YOURIMAGE to the name of your image (and path, if images aren't stored in the same path as the other game files.)
With CSS, "contain" will fill as much space as possible with your image without cropping it. If you want to fill the whole space, but have part of your image cropped, use "cover" instead of "contain".
And, of course, if you're using an image that's much smaller than the space you're trying to fill, the quality will degrade as you stretch it.