For graphical changes, your best bet is the standard set of image editors: Photoshop or Paint Shop Pro. On the free side of things,
the GIMP is a decent alternative. I haven't done much on the graphics side of things (and can't at the moment, in fact, as a RAM failure on my laptop has made my copy of WW inaccessable), but I've found the best results by using Paint Shop Pro and the GIMP interchangeably, whenever one had a needed feature the other lacked.
As for the data side of things, there's absolutely nothing in the way of easy editors or toolkits. Just dive in with your favorite text editor and start tinkering. The modding guide on Digital Eel's
website is an excellent (although unfortunately incomplete) resource.
The best advice I could offer for data editing is to find something from the base game that does something similar to what you want and alter a copy of that rather than building things from scratch. Just dig through it all and try to get a feel for how everything works. Most of it is more or less intuitive as long as you've got the mod guide close at hand.
Good luck with your efforts. I'd love to see more activity show up in here.