Prior to the 1.7 update, the 'MOJANG' loading screen texture could be edited (assets\minecraft\textures\gui\title\mojang.png) with resource packs. after the update, it cannot. it uses the default, unless you edit the one inside minecraft.jar. I know the cause of this. it is the new resource pack system that allows multiple at once. for some reason they decided that the 'MOJANG' screen should now load BEFORE textures have been loaded. I don't see why this is, since it was fine before the 1.7 update.