Is a silhouette not a "picture"? Perhaps "picture" isn't the best term to quibble over, since it is quite broad (arguably its primary use is referring to paintings or drawings).

But if we instead quibble over the term "photograph," I'd argue that a photograph of a silhouette of a mountain is absolutely a photograph of a mountain. Similarly, I'd argue that X-ray photography is indeed photography.

Its a really interesting question.

Lets take it to its farthest extent: can you take a picture of a black hole?