Add general purpose fallback image #1459
Description
In our current site, we have an SVG that's used as a fallback image by our service worker when an image request fails: http://cloudfour.com/wp-content/themes/cloudfour/static/sw/blank.svg
What's unique about this image is its responsiveness. If you scale the size of its container, you'll notice that portions of the image repeat or fix to the side rather than squashing or stretching.
(According to Git logs, I am the one who made this. I had zero memory of this until @calebeby showed me. 😳)
We have a very similar set of fallback feature images that @AriannaChau designed, and I like the look of those better. But they are not responsive in the same way.
We should craft a new image that's more consistent with those feature images but fulfills the purpose of the service worker fallback. It should have the same responsiveness as the current one (linked above), which is why I've labeled this issue as both design and dev.