Alright, tried the game with the... ehhhh... "textures" that appear in my previous post.
Holy mother of jesus, what have you done to my game.
Also, my new player:
EDIT:I think I understand what's wrong now.
in Documents\Blockland\config\client\brickTextures, there are a bunch of folders containing the textures that we download.
What's wrong is that all of the .png files are unreadable for some reason.