From what I've gatherd from helping out with uix, they're packed into the xbe.... but that doesn't seem right. I've also heard that they get put into each emulator/apps' Tdata or Udata folder.... I think it's Udata.
Yeah my bad, TitleMeta.xbx, TitleImage.xbx and the rest of them in main save folder in Udata.
You can edit the xbx easy. Fastxbx2img2dds is probably the easiest way. You can read my tutorial in the old tHc forum inside the uix forum on how to make them in depth if you need the help.