В GuiMainMenu фон выводится при помощи Tessellator:
mc.getTextureManager().bindTexture(new ResourceLocation("mod", "textures/gui/background.jpg"));
Tessellator tessellator = Tessellator.instance;
tessellator.startDrawingQuads();
tessellator.addVertexWithUV(0, height, 0, 0.0, 1.0)...