Go into the editor and then select that you will be changing the rules. Under the natural resources tab you can add new resources, edit what they do, and then select the icon you want to appear. You can then if you want add a description for your new resource in the civilopedia. Just plopping new graphics into the folder wont work.
If you need more help just ask. I also think there is a guide to adding new resources at the top of this forum, there are few if any differences between adding resources with PTW and standard Civ3.