You will need a .png for the texture you want to appear and a .properties file in order to make the game use your texture. You would have to place them both in YourResourcePack/assets/minecraft/optifine/cit .
For example, for a custom fortune III diamond axe my properties file would look like:
If you're animating the texture then you can use Minecraft's system of .mcmeta files for frame order and timing. A moderator on this forum has an excellent tutorial for Minecraft 1.6 that is still applicable.