The problem with raster images is that when you upscale it, it will lose its quality. So when you choose your raster images always make sure that their resolution is over 2000px as most of the high end modern devices are of heavy resolution. From there, reduce the size of the image to fit the lower resolution screens. When you reduce the resolution of image, you don’t lose its quality as much as when you upscale it. I hope this helps. :)