I'm painting the new release, and I can't quite tell -- are the Temple Guardians supposed to be made of stone, or are they giant snake people?

Snakes I believe. The card game has images I think.

I wish FFG would have painting guides for some of the MoM miniatures, or show painted figs on the product's webpage. Of course in the Cthulhu Mythos, your imagination/insanity can run wild! ^(;,;)^

It is hard to tell. The model looks like it could be a living statue -- the tail is kind of broken off and there's a chip or two in the abdomen. And the expansion description references statues coming to life.

But the punch-out that inserts into the base shows a green snakey-looking dude.

I'm splitting the difference and doing an Army Green base coat and dry brushing it in granite grey. Hopefully it will look good either way, given my limited painting skills.

It would not be out of place since most Meso-American cultures (and even ancient western ones like the Greeks and Romans) painted their statues to be life-like. We only see the marble because the paint has flaked off or was removed unknowingly.

I think the temple guardians are animated stone statues.

They are animated stone and hit like animated stone.