The problem is that most graphics in the game are compressed, with very few exceptions, like hitpoints or the princess room torch.
The compression algorithm of the sprites is known and documented here: https://www.princed.org/w/index.php?title=SNES_format#Sprites
And a tool to extract sprites is also available (which is written in PHP): https://forum.princed.org/viewtopic.php?f=76&t=4236
But no tool to import sprites has been created.
My modifications are intended to be mainly some color swaps/clothing additions for the Prince, Princess and Amazon graphics, for example:
Can a tool be created with all this information? Or what should I do?