It would be really nice if we could upload or set a link to an image as it is possible with the large icon option. But I’ve understood that the small icon setting can only happen if the image is in your app package, unfortunately.
It would be possible if we could upload images to res/drawable directory as we can upload to assets. As it is not possible, I see that, nowadays, decompiling/recompiling apks is easier due to existing tools, so I believe it’s worth learning the procedure.