This page provides direct links to in-game modifications. The modifications themselves won't alter the gameplay, so you can use them without fear of breaking the game.
Embodiment of Scarlet Devil
Retexture patch ~ ver 0.3
Created and packed by Emarrel
The patch replaces all the images in EoSD with cleaner, higher resolution ones. The sprites have also been updated with those from the newer games. Read the readme before installing, as it contains some important information.