How can I make a mod that adds my own paintings or decorations?

Change up how your blocks look! You don't even need to add code, just add as many Resource Packs as you like.

Choose the block you'd like to change:

https://www.tynker.com/minecraft/blocks/

All the blocks of that type in your world will take your new color or design! You can even add your favorite pixel art to a block, so you can hang it up like painting in your Minecraft house. A Minecraft block is 16 × 16 pixels wide, so you have to be efficient when you draw!