Currently, the splashscreen plugin on Windows Phone only supports a single SplashScreenImage.jpg file, and scales that to fit all device resolutions. But, it is possible to detect the resolution of the device, and allow for pixel-perfect splash screens for each individual resolution.
I have some local changes that I will send as a PR to the plugin that allows one to specify three different images, each of different resolutions to match those supported by WP8.
Resolution Dimensions File name
WVGA 480 x 800 SplashScreenImage.screen-WVGA.jpg
720p 720 x 1280 SplashScreenImage.screen-720p.jpg
WXGA 768 x 1280 SplashScreenImage.screen-WXGA.jpg