To avoid that complexity I sometimes use a single image for all devices, with high resolution and relatively high level (=low bitrate, low quality) of jpeg comp

To avoid that complexity I sometimes use a single image for all devices, with high resolution and relatively high level (=low bitrate, low quality) of jpeg compression. Modern GUI frameworks and browsers use high-quality GPU-based scalers, downscaling an image on a low-DPI device is almost free and gets rid of JPEG artifacts. On high DPI screens pixel density gets rid of these JPEG artifacts as well, unless using a magnifier, glass or software.