We dont have one big texture for all nodes but one texture for every single node. So we have e.g. default_dirt.png and default_stone.png
You can see the textures that are used in games/common/mods/default/textures/ (in Minetest 0.4.6). To create your own texture pack, you make your textures and place them in textures/all/ with exactly the same name as in the default mod.
More info:
http://forum.minetest.net/viewtopic.php?id=1592http://wiki.minetest.com/wiki/Texture_Packs