Here's a tutorial for sprite editing (it has everything):

http://gbatemp.net/topic/306115-how-to-make-edit-and-insert-sprites-for-pokemon-black-white/

as for the Gym Leader 'sprite numbers', you'll have to find those yourself. Other hackers had to find it themselves.
BTW it's not that hard finding them, just scroll through the images with the hacking tool.
(You'll know when you use the Sprite Tool)

 

Try asking the leaders of the other ROM hackers for help. (Ex. Hoenn White)
Never mind about the Over-world Sprites then.