Easier way to scale assets for various iPhone sizes?

When creating apps, is there any easier way to scale images, aside from creating and loading different assets based on the detected device? It just seems kind of ridiculous to have to go through the pain of creating different images for iPhone 6+, 6, 5, 4, and also tweaking various positioning on the screen based on the device.

Also, I'm using Spritekit particle effects, and those have to be re-created too for different devices.

I know you can just have Xcode upscale iPhone 5 assets, but obviously the trade-off is image sharpness. It would be nice if you could just design everything for the largest iPhone, and have it scale down automatically for other devices. There isn't such a way, is there?