Thank you for your reply! I'm not good at this, but I've been able to find a community of people who are smarter than me. The question of changing the intro to a custom one is now under consideration by these talented people. I think something...
If there is no check over the mentioned file you just need to replace the audio and png bytes inside the file taking care of maintaining the correct file size. Anyway I never modded it by myself because I was simply not interesd in doing that :P