FEW QUESTIONS- not sure if right spot

i figure what better place to ask these questions then my favorite rom site.
A friend of mine recently downloaded a...i gues homebrewed pokemon lol. they changed the little pokemon creatures into little anthromorph creatures still sprites and all. Nothing of the game was changed OTHER then the pokemon images.
I was like how can someone do that?!!! so that is what i want ot ask..how do you do that? is there a way to extract the files of a ds rom and switch the images inside with my own made ones??? is there a way icould copy the pokemon program..and just add my own images useing the ds game maker???
Which leads me to the next question. people who make their own ds games..is dsgame maker a good one or is there better? i read about NO$GBA but..i still cant figure out what it is exactly
THANK YOU SO MUCH FOR THE HELP
sandy
p.s. im not wanting to change the pokemon for profit...it mearly something i would like to experment with for my own enjoyment. cause i like to make sprites so it would be neat to make them for games i like and just change them out.like when the hero is less desirable adn i rather have bunny ears lol.
thank you again

Hacking the ROM, there's quite a few tools for Pokemon especially, like at pokecommunity.com etc. They're usually GUI tools,so it's really easy to do. DS Game Maker can't edit already made ROMs, it's gay anyway, so don't use it. no$gba is the best NDS emulator for your PC, so you can play NDS games on your PC. DS Game Maker is a very budget GUI application designed to easily add PALib tools, PALib is a NDS library which allows you to make DS games. You still can't edit previously made ROMs with of these, or easily, they require hex editing and whatnot.