Module Creation and custom graphics

Hi, I've been meaning to get into custom module creation and I wanted to ask a few things about graphics. As I understand one can almost completely overhaul the graphics of the game by adding thier own sprites for characters and monsters and their own tyles for the maps. I would like to ask a few questions if someone would be willing to help.
1 Are there any limitations to this?
2 How exactly can this be done?
3 Will the custom graphics I make for my module also show up for everyone who plays it?
4 can one even replace existing creatures with custom sprites?
1 Are there any limitations to this?
2 How exactly can this be done?
3 Will the custom graphics I make for my module also show up for everyone who plays it?
4 can one even replace existing creatures with custom sprites?