I use a resource pack to add custom textures and other assets in MCP; as long as it is selected the game will attempt to load assets from there first so there are no errors and transferring them to the final reobfuscated mod/Minecraft jar is as simple as copying the assets folder from the resource pack as it has the exact same layout as the assets in the jar.
Otherwise, there is an assets folder located at MCP\jars\assets, though I haven't tried changing it other than to fix the default sound/icon assets (it need to be changed so all the stuff in "assets\virtual\legacy" is in the root of "assets"; note that this probably only applies to 1.6.4 and older versions, which use legacy assets, and either way it is preferable to use a resource pack over modifying the existing MCP assets as it is much easier to manage than having your assets mixed with the vanilla assets).
let say i made Ruby block, where is the assets folder to put my textures on it??
I use a resource pack to add custom textures and other assets in MCP; as long as it is selected the game will attempt to load assets from there first so there are no errors and transferring them to the final reobfuscated mod/Minecraft jar is as simple as copying the assets folder from the resource pack as it has the exact same layout as the assets in the jar.
Otherwise, there is an assets folder located at MCP\jars\assets, though I haven't tried changing it other than to fix the default sound/icon assets (it need to be changed so all the stuff in "assets\virtual\legacy" is in the root of "assets"; note that this probably only applies to 1.6.4 and older versions, which use legacy assets, and either way it is preferable to use a resource pack over modifying the existing MCP assets as it is much easier to manage than having your assets mixed with the vanilla assets).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?