Fixed it.
It should be:
static const struct SpriteFrameImage sTrainerBackPicTable_Wyatt[] =
{
{gTrainerBackPic_WyattTiles, 0x800, 0},
{gTrainerBackPic_WyattTiles + 0x0800, 0x800, 0},
{gTrainerBackPic_WyattTiles + 0x1000, 0x800, 0},
{gTrainerBackPic_WyattTiles + 0x1800, 0x800, 0},
}...