The best solution in this case is for Construct 2 to download images layout-by-layout, and not try to hack the same thing in using 'load image from URL'. (C2 already loads textures in to memory layout-by-layout, but it still downloads them all on startup.) This is on our todo list, but I can't give an ETA.
For some users with good broadband connections, even a 400MB game won't take that long to load - if you're maxing out a good connection, it will only take a couple of minutes (which you can pass more easily using a custom loader layout). They'd have to wait equally as long, and without your own loading screen, if they downloaded a native app.