You can edit/add:
dev-version> ./data/textures/block//002.png
current-version> ./data/textures/block/002.png
./data/image-resource/shield_tex.png for build mode mirrow-layers and shield.
./data/image-ressource/... for how the ingame menu looks (just view the default pngs and guess what they are for)
./data/config/blockconfig.xml - it says what blocks can do on localhost. Multiplayer-servers overwrite it with their own config (install the required block-textures)
You can also edit ./data/sky/* - just keep the names, file type and size.
What do you mean with mod?