In Minecraft, there are things called Resource Packs. You may have seen other people play Minecraft, and they have the textures are different. This is done with Resource Packs. A resource pack changes the games textures, and are made by players.
You can just edit the Minecraft textures, and put it into a zip file. You can edit not just the textures, but even the sounds! Some people like the original textures, and I must say, I am one of those people. But sometimes it is fun to have cooler textures, and I was thinking of adding a very high resolution one to my Minecraft. Normally, a side of a Minecraft block is 16 pixels by 16 pixels.
But you can get HD ones that are up to 520 by 520 pixels! Anything above 16 by 16 requires to be manually packed into a special .jar file, but Optifine helps with that. You just need to put it into the resource packs folder, and select from the Resource Packs screen in Minecraft. Apparently, in the new 1.
8 update, you will be able to edit the shapes of blocks too, making them smaller or bigger!