In that case, simple indexing should work fine:
N, H, W = 12, 10, 10
landmarks = torch.randint(0, H, (N, 2))

tmp = torch.zeros(N, 3, H, W)
rows, cols = landmarks.split(1, 1)
tmp[torch.arange(N), :, rows, cols] = 1.

image = torch.randn(N, 3, H, W)
image = torch.cat((image, tmp), 1)
print(image.shape)
> torch.Size([12, 6, 10, 10])